Como ATB observa em um comentário sobre a questão, os Valar não querem afundar Beleriand. Acontece que Morgoth era tão poderoso que a destruição de Beleriand foi uma consequência inevitável de derrotá-lo 1 ; somos informados em O Silmarillion que:
Portanto, a razão pela qual Ossiriand é poupado é bastante simples: Morgoth não tinha forças significativas em Ossiriand; na verdade, nos disseram isso também, após o cerco de Angband (ênfase minha):[S]o great was the fury of those adversaries that the northern regions of the western world were rent asunder, and the sea roared in through many chasms, and there was confusion and great noise; and rivers perished or found new paths, and the valleys were upheaved and the hills trod down; and Sirion was no more.
The Silmarillion III Quenta Silmarillion Chapter 24: "Of the Voyage of Eärendil and the War of Wrath"
[The Orcs] passed over Gelion with fire and terror and came far into East Beleriand. Maglor joined Maedhros upon Himring; but Caranthir fled and joined the remnant of his people to the scattered folk of the hunters, Amrod and Amras, and they retreated and passed Ramdal in the south. Upon Amon Ereb they maintained a watch and some strength of war, and they had aid of the Green-elves; and the Orcs came not into Ossiriand, nor to Taur-im-Duinath and the wilds of the south.
The Silmarillion III Quenta Silmarillion Chapter 18: "Of the Ruin of Beleriand and the Fall of Fingolfin"
1 Eu discuto as razões para isso um pouco mais completamente na minha resposta para Por que o Valar abandonou o Moriquendi e os homens?