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.
I just watched some youtube videos about amputee dogs, including this one: http://www.youtube.com/watch?v=iJxEIXRz_Kk
This was the first one I found that had any information about the dog's reaction after the amputation, and much later. It says the dog took 4 weeks to "start acting like himself," and still whined at night, 6 weeks later. This seems about the same timescale as humans adapting to disabilities, so you're right about hedonic treadmills in dogs. Probably a lot of other animals have them too. There's still all the animals that don't hav...
Assume for the time being that it will forever remain beyond the scope of science to change Human Nature. AGI is also impossible, as is Nanotech, BioImmortality, and those things.
Douglas Adams mice finished their human experiment, giving to you, personally, the job of redesigning earth, and specially human society, according to your wildest utopian dreams, but you can't change the unchangeables above.
You can play with architecture, engineering, gender ratio, clothing, money, science grants, governments, feeding rituals, family constitution, the constitution itself, education, etc... Just don't forget if you slide something too far away from what our evolved brains were designed to accept, things may slide back, or instability and catastrophe may ensue.
Finally, if you are not the kind of utilitarian that assigns exactly the same amount of importance to your desires, and to that of others, I want you to create this Utopia for yourself, and your values, not everyone.
The point of this exercise is: The vast majority of folk not related to this community that I know, when asked about an ideal world, will not change human nature, or animal suffering, or things like that, they'll think about changing whatever the newspaper editors have been writing about last few weeks. I am wondering if there is symmetry here, and folks from this community here do not spend that much time thinking about those kinds of change which don't rely on transformative technologies. It is just an intuition pump, a gedankenexperiment if you will. Force your brain to face this counterfactual reality, and make the best world you can given those constraints. Maybe, if sufficiently many post here, the results might clarify something about CEV, or the sociology of LessWrongers...