Sauron não podia imaginar que alguém pudesse suportar o poder

É declarado no Conselho de Elrond, como outros métodos de eliminação do anel são levantados, quando Gandalf faz questão de que Sauron nunca pensaria em alguém querendo destruir o anel, por causa do grande poder que possui:

"Into his heart the thought will not enter that any will refuse it, that having the Ring we may seek to destroy it. If we seek this, we shall put him out of reckoning."
_{The Fellowship of the Ring - The Council of Elrond}

E Sauron estava certo: mesmo para Frodo, que havia carregado o anel de inúmeras ligas, a tentação no final era muito grande pouco antes de sua destruição, e sem a ajuda de Gollum, a Irmandade não teria tido sucesso.

‘I have come,’ [Frodo] said. ‘But I do not choose to do what I came to do. I will not do this deed. The Ring is mine!’ And suddenly, as he set it on his finger, he vanished from Sam’s sight.
_{The Return of the King}

Finalmente, como indicado com excelência por @Pryftan em seu resposta encontrada aqui , Tolkien escreve, na famosa Carta 131, que Sauron não precisava usar o anel para manter seu poder a menos que fosse reivindicado por outra pessoa. Continua descrevendo como ele estava convencido de que ninguém poderia destruir o anel antes de sucumbir a ele. (O que mostramos acima é verdade).

But to achieve this he had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place. This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and his desire to establish control over the minds and wills of his servants. There was another weakness: if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being would be diminished to vanishing point, and he would be reduced to a shadow, a mere memory of malicious will. But that he never contemplated nor feared. The Ring was unbreakable by any smithcraft less than his own. It was indissoluble in any fire, save the undying subterranean fire where it was made - and that was unapproachable, in Mordor. Also so great was the Ring's power of lust, that anyone who used it became mastered by it; it was beyond the strength of any will (even his own) to injure it, cast it away, or neglect it. So he thought. It was in any case on his finger.
_{The Letter of J.R.R. Tolkien - Letter 131, to Milton Waldman}

Tolkien claramente descreve que Sauron foi incapaz de perceber que alguém passaria por Mordor, subindo ao lado de Orodruin e entrando no Sammath Naur sem ter cedido às tentações do anel. Isto é mais provável porque ele nunca conheceu Hobbits nem foi capaz de prever sua falta de desejo por poder e seu conteúdo por uma vida simples.

Tolkien fala sobre isso na Carta # 131. É uma carta bastante longa, mas a parte relevante é assim (estou digitando isso e, assim, qualquer erro é minha culpa).

[Além disso: Tolkien explica primeiro como Sauron se tornou quase supremo na Terra-média usando o Um Anel: ele, afinal, dominou multiplicando hordas de Homens que não tinham contato com Elfos e tinham um império em crescimento. Então ele - Tolkien - continua como abaixo]

But to achieve this he had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place. This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and his desire to establish control over the minds and wills of his servants. There was another weakness: if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being would be diminished to vanishing point, and he would be reduced to a shadow, a mere memory of malicious will. But that he never contemplated nor feared. The Ring was unbreakable by any smithcraft less than his own. It was indissoluble in any fire, save the undying subterranean fire where it was made - and that was unapproachable, in Mordor. Also so great was the Ring's power of lust, that anyone who used it became mastered by it; it was beyond the strength of any will (even his own) to injure it, cast it away, or neglect it. So he thought. It was in any case on his finger.

Em um comentário para outra resposta, notei que se Sam tivesse entendido melhor o relacionamento de Gollum e Frodo ele os teria ajudado a entrar em Mordor e depois de ter tomado o Anel de Frodo jogou-se no fogo. Esta é uma parte de uma carta que explica isso. Como um bônus adicional, tem outro bocado que explica mais uma vez como Sauron não achou que alguém estaria disposto a destruir o Anel.

If he had understood better what was going on between Frodo and Gollum, things might have turned out differently in the end. For me perhaps the most tragic moment in the Tale in II 323 ff. when Sam fails to note the complete change in Gollum's tone and aspect. 'Nothing, nothing,' said Gollum softly. 'Nice master!'. His repentance is blighted and all Frodo's pity is (in a sense) wasted. Shelob's lair became inevitable.

This is due of course to the 'logic of the story'. Sam could hardly have acted differently. (He did reach the point of pity at last (III 221-222[4]) but for the good of Gollum too late). If he had, what could then have happened? The course of the entry into Mordor and the struggle to reach Mount Doom would have been different, and so would the ending. The interest would have shifted to Gollum, I think, and the battle that would have gone on between his repentance and his new love on one side and the Ring. Though the love would have been strengthened daily it could not have wrested the mastery from the Ring. **I think that in some queer twisted and pitiable way Gollum would have tried (not maybe with conscious design) to satisfy both. Certainly at some point not long before the end he would have stolen the Ring or taken it by violence (as he does in the actual Tale). But 'possession' satisfied, I think he would have then sacrificed himself for Frodo's sake and have voluntarily cast himself into the fiery abyss.

I think that an effect of his partial regeneration by love would have been a clearer vision when he claimed the Ring. He would have perceived the evil of Sauron, and suddenly realized that he could not use the Ring and had not the strength or stature to keep it in Sauron's despite: the only way to keep it and hurt Sauron was to destroy it and himself together--and in a flash he may have seen that this would also be the greatest service to Frodo.** Frodo in the tale actually takes the Ring and claims it, and certainly he too would have had a clear vision - but he was not given any time: he was immediately attacked by Gollum. When Sauron was aware of the seizure of the Ring his one hope was in its power: that the claimant would be unable to relinquish it until Sauron had time to deal with him. Frodo too would then probably, if not attacked, have had to take the same way: cast himself with the Ring into the abyss.

[4] Esta é uma nota de rodapé da própria carta; Eu não percebi isso até depois de tirar uma foto, mandar por e-mail para mim e estar transcrevendo. Acho que não preciso mencioná-lo no entanto.

Porque Sauron criou o Um Anel para exercer poder sobre os outros anéis ("Um Anel para governar todos", etc.)

As pessoas tendem a ver o mundo por meio de seus próprios filtros. Se alguém acha uma determinada coisa desejável, muitas vezes espera que outras pessoas a considerem desejável também. Isso é uma espécie de natureza humana.

A idéia de que alguém possa encontrar o Um Anel e escolher destruí-lo é completamente um anátema para ele. Isso seria rejeitar o poder que ele contém - algo que o próprio Sauron nunca faria - e por isso ele tem dificuldade em imaginar que qualquer outra pessoa também possa fazê-lo.

'Strange powers have our enemies, and strange weaknesses!' said Theoden. 'But it has long been said: oft evil will shall evil mar. '

Por que as pessoas que derrubam tiranos muitas vezes se tornam (sem dúvida, piores) tiranos?

Não é que o Anel corrompa insidiosamente qualquer um que o tenha tomado, embora de fato faça isso. É exatamente como Gandalf diz: o próprio Sauron era governado por seu próprio desejo de poder, e simplesmente não podia compreender que alguém, dada a oportunidade de poder, não o desejaria da mesma maneira que ele. No entanto, no LOTR, vemos um número de pessoas fazendo exatamente isso. Não Bilbo ou Frodo, na verdade, que não podiam usar o poder do Anel, mas outros como Tom Bombadil, Elrond, Gandalf e Galadriel, que provavelmente poderiam tê-lo, mas preferiram não aceitá-lo.

Outros já explicaram bem porque o pensamento nem sequer ocorreu a Sauron, destruir a coisa mais poderosa e preciosa do mundo seria um conceito totalmente estranho para ele.

Uma coisa importante a notar é que a história prova que ele está certo em não considerá-la, porque mesmo que ele considerasse a ideia, ele a dispensaria porque ninguém pode destruir o anel . Mesmo alguém strong o suficiente para suportar seus poderes sedutores como Frodo (de quem é dito explicitamente "Se você não pode fazer isso, ninguém mais tem uma chance") se rompe completamente quando confrontado com a tarefa de realmente destruí-lo.

Existem apenas dois casos de alguém desistindo do anel por vontade própria, um deles foi Bilbo, que teve que ser persuadido a fazer isso com muita força por Gandalf, e o outro foi Sam em Mordor quando o devolveu a Frodo. . E, pelo menos na minha opinião, é altamente duvidoso que Sam tenha tido a força para realmente jogar o anel no vulcão no final se ele tivesse chegado lá por conta própria.

E Sauron sabe quão sedutor é o anel, porque ele fez isso.

Se você pensa assim, a questão é: "Por que Sauron acha que ninguém tentaria fazer algo que é simplesmente impossível?", eu acho que a questão se torna bastante clara, até mesmo esclarecida pelo fato de que ele está obviamente correto, nenhum ser no mundo teria a força de vontade de lançar o anel no fogo.