You said the quality of life is better for animals in zoos because they're given food instead of having to hunt and they don't have to fight for territory and mating rights.
Animals want to hunt and they have to release their aggression somehow. With no use for the fight/flight response in captivity, they're left with high levels of unspent energy which is when you get depression and antisocial, self destructive behaviour. A good zoo will do its best to let animals display their natural behaviours, and that includes fighting for dominance, space, food and mates. Take that away from them and you're taking away a large part of their reason for being.
Of course, to do this you need massive enclosures, like you said. Enclosures that simulate their natural home ranges, which can stretch for miles depending on the animal. It's just not possible for any zoo to provide such space.
Maybe in Utopia, it could happen. But in Utopia we wouldn't have poaching savages and massive-scale deforestation anyway so there'd be no need for zoos.
I don't even know if I responded to your post properly... I'm aching all over and it's affecting my mind.
Like I said, it depends on the animal. There are two situations we are talking about here.
1.) Animals would have food and things. This is in reference to rehabilitation and animals that would have died. its obviously preffereable for an animal to be in its natural environment, the point is that said environment does not always allow that animal to live, in which case its better off in a Zoo where it can be provided what it needs.
2.) the second situation pertains to zoos that do not rehabilitate (which at least where i live is a minority). I am still okay with these zoos existing, provided that the animal does not know or not is negatively impacted from living in the zoo. You are right that in some situations and with some animals this would require huge amounts of land. But if the zoo cannot provide this then it should not have it. And if the zoo did provide these things the animal would be better off, as it would have protection from poachers, access to vets, and a steady supply of food that it could naturally obtain (be it hunting, foraging, or what have you). There are zoos that specialize in what i have just described, its not only a Utopian idea. Is it most zoos, no, should it be, yes.
This is why in many cases (i said not all) the quality of life is better in a zoo, even if its far from perfect. Of course if a zoo has an animal that it cannot recreate the environment for its going to be an unhappy animal, but that's just a bad zoo, and should not negatively impact our impression of all zoos.