I don't think anyone is offended.. and lots of folks do keep fish together that in the wild would never share the same waters and the fish appear to be fine. But we can't ask the fish what they think about the tank they're in either, and in a lot of cases the life span they have may not be what they could have had. But in a tank, they are unlikely to be preyed upon, whereas in the wild, fish are prey for all kinds of things.
For me, it's simply that as I have grown older, and have learned more, I have concluded that it is better to practice husbandry for pets so that they live a decent life that's as close to what they would have had in in the wild as is practical. One example would be that I used to feed my cat dry kibble. Now I know cats are obligate carnivores, so I feed a raw meat diet. The difference to the cats has been nothing short of amazing. I find the same difference in my fish when I feed a lot of live food instead of all man made food. That's a choice I decided to make.
Because we keep our wet pets in tanks, they can't just up & swim away if the conditions aren't quite what they'd prefer. But in fact, we can't replicate what they had in the wild; not even huge public aquariums can do this, though they can do it better on their scale than most hobbyists could. But I think it's a good thing to at least try.
It has long been said that fish are very adaptable, and I think many of them really must be, because they do survive in waters that are vastly different from what the species came from. But all we know about many species local conditions, in most cases, can often be based on rather small samples.
The folks who study them go fishing, literally. Fish are caught and if they're interesting or new species, whoever is studying them will test the water they came from to find out what the parameters are. They'll observe the type of substrate, plant life, water movement, temperature, etc., and try to classify the fish according to the rules for that sort of thing. Eventually this information will be published. This is what is used to provide the recommendations as to what a given fish prefers. There are more than a few fish in the hobby who only come from one tiny area in one lake or one river, and then are others who are much more widely spread. Sometimes they'll find a species later on in other places, and maybe that will indicate it can thrive in more than one set of conditions. But it's the conditions that exist at the time the fish is caught, not even what might exist during different seasons.
Once a species has been kept captive for awhile with reasonable success, whatever appeared to work with that species also becomes a source of recommendations for how to keep them.
So a lot of fish are pretty adaptable, because if they were not, a great many of us just wouldn't have many choices for fish to keep, unless we were wiling to do a lot of water modification.
I guess my take on it is that just because the fish CAN do it, isn't necessarily the best reason to make them do it. So that's what I have chosen to do, based on what I've learned so far.. and I keep learning more as time goes on. I hope I never stop.