It seems like we have two disagreements. The first is whether there are living conditions to which death is preferable, and the second is over how bad the conditions wild animals live in are.

I agree that there are circumstances to which death is preferable, although I've argued a number of times on this site that people who're making that decision with respect to themselves are usually in a bad position to do so.

I strongly disagree that the conditions on wild animals are that bad.

There's a very strong selective pressure for animals to be adapted to their own specific living circumstances. Animals can certainly become upset or depressed when removed from circumstances they're comfortable with, witness the preponderance of zoo animals whose habitats aren't made sufficiently reflective of what the animals would have to live with in the wild. They often become visibly depressed or neurotic, despite living much safer, physically healthier, and longer lives.

As for the hedonic treadmill, if a human is hit by something they have a low chance of surviving, they're probably not going to survive. That's tautological. But that doesn't mean that practically any injury an animal receives is probably going to result in its death. It's not as if humans have a an evolutionary pressure to be able to bounce back from ailments that other animals simply don't have.

Try watching some amputee dogs. See if they seem so miserable.