If someone is vegetarian/vegan for ethical reasons, it's because they want there to be fewer factory farms. This would of course mean that the animals they would have eaten wouldn't exist at all, so of course they'd consider nonexistence better than a life of torture.
"I'd rather you not have lived at all, rather than live a life of torture" is a pretty mainstream opinion in general, I think.
Unless cloning counts? Is extinction okay as long as we have some copies of their DNA and plan to clone them into a decent life eventually? I feel rather uneasy about that resolution; at the very least I'd want to see a species successfully reconstructed in this manner before giving the possibility moral weight.