As much as we rant and gnash our teeth at the poor LFS stock, the overbred state of some of the most popular common fish, the rampant diseases that decimate our tanks --- I (and many others!) have come to the conclusion that it is really like 85 to 95% of all unnatural (i.e. not simply old age) fish deaths are from poor husbandry. As has been said often above, we have to try our best to replicate nature, not a trivial task! And poor husbandry = stressed fish = more susceptible to disease. So, I'll be the one who says that it is mostly not the fishes fault, it is our fault.
The reason only a few percent of wild fish survive is that they play a significant role in the food chain. Most aquarium fish are just feeders in the chain in their natural eco-system -- if the fish were near the top, they would never be able to be kept in an aquarium. They have to lay 100s to 1000s of eggs, because their whole role is to be eaten by the larger fish, so heck yeah they are not going to survive. But, as aquarium owners, we have not replicated the entire eco-system, so most of our fish should not have to worry about being consumed. There will always be a few sickly or weaker fish, that is the whole idea of genetic diversity, and correspondingly there will always be a few stronger or hardier fish. But, given good conditions a great majority of the fish should be more than fine.
Yes, they can be very hardy creatures -- how many fish did we really acclimate the first time? For that matter, how many fish went through your first cycle?
But, if you want them to live out their long natural life, and at least live life close to how they would in nature, you have to do the water changes matching pH, hardness, temperature, give them a tank of a decent size to some of what they would in nature, give them the rocks or plants to make them feel secure, etc., etc. Otherwise, the fish are not as stress-free as they could be, which will make them less hardy. If you have hardy fish now, and intend to start skipping on your maintenence, I would bet they are significantly less hardy in 6 months. So, why risk it?