On some level you do have a point really, but like most rationalists I believe you are over-simplifying a bit. For an example, the whole myth that "wild" animals have a less "stressful" life when in a zoo or even (proper) private ownership. The truth being that you are looking at critters that are used to roaming tremendous distances, oft times in pack or community settings -- there are no boundaries to them outside those established by nature itself. How would you react to such a loss of freedom? How stressed out do you think you would get if you found yourself "controlled"; able to eat only what your handlers gave you rather than using your natural skills and instincts for obtaining food -- a sense of actual purpose & dignity?
My situation in life is very much like that of a zoo animal -- limited! I can assure you, it's not fun. It is very painful in fact, not being able to travel as I once did; to be dependent on the medication & constant "testing" and so forth that adds to that sense of lost freedom. More pertinent to the issue here, I'm not able to mingle with "my own" because of the constrictions set on my life these days, which happens to include a long lived rapport with numerous exotic beasties (what can I say? I'm a sucker for a fury face & cold nose
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Two of the more exotic critters in my early adult life were snakes; "Tiny" a 24 ft. Anaconda and "Terracotta" a 14 ft. Burmese Python both of whom lived very long lives for their type. . . the python was well over 16 years old when we lost him to liver failure while Tiny, nearly 20, simply died from heat stroke during an engagement at the Ohio State Fair. . . we couldn't keep him cool enough during the high heat. The problem however, is that serpents of these proportions require a great deal of territory just to stretch out and get proper exercise; even a room the size of your typical Bungalow styled home's living room area is too small (technically) for such a large snake. Too, they aren't given a "choice" when it comes to prey most of the time, such as they would have in the wild and on that note, they are deliberately fed killed food vs. live so as to help "tame" them and make them more docile (large breed owners even meditate with their snakes to add to this "energy shift")
Even my wolf Khali, would consider everything within 15 miles of our home, her "natural" territory, not the 14 acres we lived on at the time and certainly not the environs of a small downtown apartment. So when she was "donated" to the U.S. Wolf Reserve and given over 400 acres to enjoy, alongside other greys & timbers, she found a kind of "heaven" for herself. Unfortunately such reserves are few and far between and worse, there aren't as many understanding and supportive ranchers in the world that are willing to work WITH such an agency.
Understand, I'm far from being one of these deluded idiots that want to pass laws forbidding any form of pet ownership and animal captivity; I understand the necessity for such things on behalf of humankind as well as the animal population and that includes the "cruel" experimentation done within the medical & science environs; while some of it is certainly redundant and no longer practical there is a tremendous amount of it that ultimately benefits both, human and critter (even in the wild). That includes special programs like the Rabies Vaccination Programs in which air-pistols & rifles are used to "dart" wild vermin (primarily raccoons, skunks and other large rodents) when an especially rampant rabies season seems to arise. The cost of such programs is outrageous, but when incorporated they have proven an exceptional benefit to one and all, giving a healthier animal population (not just the infected little dudes but in a more general manner due to reduced spread of the disease into other species). I guess what I'm pointing out here is how many of the medical benefits you mention are now available in the wild and are slowly becoming a matter of action vs. vision. This includes feeding wild animals to a certain point but only during dire periods such as hard winters or exceptional flooding, etc. Hay and even small mammals (goats, pigs, etc.) are set into key areas so as to support food needs to specific animal groups; especially when said populations involved endangered species.
Because of how the Human Cancer has infected the globe, we are becoming more and more responsible for wildlife but even with the level of effort being put out, we loose entire species on an annual level because "MAN" refuses to live within the auspices of nature and her "rules". It's a peculiar conundrum to say the least, but a median exists -- the key aspect of which centers on human being waking-up to the reality that they are the one's infecting all of the globe and thus, catalyzing -- stimulating -- otherwise dormant or even unheard-of, disease that affects ALL animals on some level as well as plant-life (but probably to a far lesser degree).