This is also true for fish like discus. Yes, they are adaptable to a wide array of water parameters, but we do large, frequent water changes due to things like bacteria loads building up in the water that will stress the fish. As you mentioned, extremely soft blackwater environments do not allow most bacteria to thrive, and so the fish are not equipped to deal with a build-up of such bacteria in an aquarium setting. In addition to replacing trace elements/minerals that are absorbed by the fish and reducing DOC/nitrate, there are other reasons for water changes that people do not necessarily see with the naked eye but that are directly linked with the environments in which the fish evolved. It's why I get so impatient with those who suggest water changes are unnecessary.Agree with qguy, it would take many, many generations to even scratch the surface and undo those millions of years of evolution.
The tannins in black water tend to have a natural antibacterial effect.
Seems as oscars age they become more and more susceptible to bacteria that cause HITH, and other maladies. The soft, tannin infused water helps to prevent this, and besides, the rivers oscars come from, move in the million of gallons per second, so even the most prolific water change regime can't compare.
We are lucky that so many cichlids are so adaptable. I am able to keep and breed a lot of the species that I do because of the resilience and hardiness of the fish, and their ability to acclimate to very different water chemistry from where they were fished.