Have enough hit points that a magic missile spell doesn't kill them? Which if you're up against someone who can "mythically quicken, empower, etc." a spell should always be the case.

I suspect many of these problems you're asking about are from just reading the rules and not from actually playing the game. In high level and mythic play, the single biggest factor is the action economy. Using an action on magic missile means you're not using that action (and spell slot) on something even worse, like every other non-first level spell. If I can do a super duper empowered mythic míssil mágico spell, or a, say, mythic persistent heightened carne em pedra, the latter is usually a better one-round killer.

I played in a Wrath of the Righteous mythic campaign and our caster had mythic míssil mágico - and used it when running low on other stuff, or when a little extra surefire damage would take out a wounded foe - in other words, whenever you'd use a normal magic missile in non-mythic mid-level play.

There's some mythic powers and stuff that can protect even against mythic spells - but that's not the real answer to this question. "Because there's something better to do with that action" is the real answer.

Talk to the GM about the broche de blindagem

The missiles created by the mítico míssil mágico spell "bypass the escudo spell and similar effects that block the non-mythic version of this spell," but the broche de blindagem, despite the name and despite using the spell escudo in its construction, does not, technically, quadra the missiles from a míssil mágico effect: "[I]t can absorb magic missiles of the sort generated by the spell or spell-like ability[, absorbing] up to 101 points of damage from magic missiles before [the broche] melts and becomes useless."¹

O broche de blindagem (1,500 gp; 0 lbs.) is, however, extremely narrow in function, can't be recharged, e occupies the wearer's neck slot, which is extremely valuable personal real estate. By using the rules for Adicionando novas habilidades to magic items, with the GM's permission, a broche de blindagem effect could perhaps be added to another inexpensive and slotless item (e.g. an pedra ioun (dull gray) (25 gp; 0 lbs.)) for 2,250 gp. (Both broche e pedra estão itens maravilhosos and, as there's no trend of magic items that absorb spells occupying the neck slot, adding the broche's effect to the pedra shouldn't be an issue; see Altering Existing Magic Items under Magic Item Slots para Descobridor's heartless limits to such effects.)

But, because using the broche to defend against mythic magic missiles is reliant on a technicality, the GM could rule (and, perhaps, equally technically, maybe correctly, seeing as how the escudo soletrar is usado no broche's creation, after all) that the broche is no defense against the mythic míssil mágico, ruling that the terms quadra e absorver are here close enough to be synonyms. This GM, though, given the lengths the creature's gone to to make itself—briefly!—immune to something as rare and weird (in this GM's campaigns, anyway) as míssil mágico efeitos seria let such a defense function, even against a mythic magic missile spell. The broche will be slag soon enough under an assault by any competent mítico míssil mágico Metamagic Master mage.²

Sobre isso mítico míssil mágico...

An habilitada (+ 2) maximizada (+ 3) quickened (+4) 1st-level míssil mágico spell cast by a creature possessing the trait Metamagic Master (míssil mágico) (−1) spends one of that creature's 9th-level spell slots. While a level 20 caster's version of such a míssil mágico spell can be cast as a swift action, it deals only 25+((5d4+5)/2) points of damage, or, on average, about 43 points of damage. That's not only embarrassing but also pitiful for a 9th-level spell slot. A similarly beefed up mítico míssil mágico deals a more impressive 90+((20d4+10)/2) points of damage, or, on average, about 120 points of damage—more impressive but ainda a 9th-level spell slot e costing of 2 uses of mythic power. Although, the line-of-sight range and ability to bypass spell resistance and even spell immunity estão excellent benefits of mythic míssil mágico. I can imagine the efficacy of such a spell being taught in wizarding classes: reliably picking off foes on the horizon is pretty cool, and for a caster with the correct trait, the spell's only 5th-level if it needn't be quickened (the mythic power cost will limit its usefulness, though).

So while your party's warrior can worry about this spell, the party's unlikely to encounter the spell a lot unless the GM routinely punishes players by dealing their PCs (45+((10d4+10)/2)) points of damage seemingly out of nowhere by sticking Vôo, maior invisibilidade-using mythic tier 4+ wizard 9s on every horizon.

¹ I must add here that I really enjoy authors making absolutamente certo the reader knows what a particular rhetorical flourish na realidade means in game terms, like melts e becomes useless so that there's no argument about a melted broche continuing to suck up magic missiles. My favorite example of this is from ADICIONAR's Monster Manual (1978): "If a [mind flayer's] tentacle hits it will reach the opponent's brain in 1-4 melee rounds and draw it forth, immediately killing the creature" (70), y'know, just in case a player at the table argues, "Hey, I'm a fighter! What do I need a brain for!?"
² Probably far fewer creatures deveria have the trait Metamagic Master than do. The regional trait originally appeared in the Dragon Empires Primer as Wayang Spellhunter (14), and, as the name and provenance suggests, a GM may limit the trait's availability to those from a region like Golarion's Minata which is populated largely (but not exclusively) by wayangs.