Fish have become more accustomed to higher temperatures as well through the years so that kind of rule should be continuously re-evaluated and adjusted in a changing climate, as appropriate.

Thats not how biology works. With sweet water, oxygen supply is the killer at high temperatures, and as soon as more than a few fishes die, a cascade reaction is triggered. Yes over centuries, more resilient species might emerge, but short term, life in a body of water can just collectively die. There are very frequent events in the summer, where small lakes completely die, if the point of no return is crossed.