Good point and examples but this cannot be entirely applicable to every fish, who possess varying degrees of adaptability.I keep a lot of fish from Uruguay (which is varying degrees of sub-tropical) and providing a winter cool down period is essential to the long-term health of the fish.
I've tried keeping Gymnogeophagus and others in low to mid-70s (the low end of tropical temps) all year and found that the fish would break down and die inexplicably, even when all of the other water parameters were ideal. Cichlids should live more many years and goonches probably decades.
I now keep Uruguayan fish on the bottom and floor-level tanks in my basement fishroom (without heaters). The water gets down to the mid-50s in the winter and the fish come out of the cool season seemingly refreshed and ready to spawn. The fish grow slower but I have much lower mortality and seemingly unexplained, weird health issues....
For example an RTC: young fish live in the shallows, in warmer waters. Adults are found almost exclusively in the deep, murky main Amazon channels wherein the water temp on the surface is ~80 F and higher whilst divers report that 10'-15' below the temp plunge to low seventies and lower.
Also, the Amazon is fed from the Andes glaciers - vast water masses enter the river at 38 F. IDK how long/what distance it takes for the water to warm up and to what degree but being aware of this may help further thinking and researching. Asian rivers are fed from the very high mountainous regions too.
The RTCs probably have not been kept long enough for the last thought but FWIW, the oldest RTC's in captivity I know of were two 27 year olds of Taksan's that died of what appeared as natural causes. Many fish pros and fish heads believe RTCs can hit 50-100 years in the wild. Perhaps because they experience lower temps.