Lifespan is a good factor indetermining quality of life, but not the only. Just because someone's oscar lived in a 40 for 15 years doesn't mean it was any good for them. For the most part lifespan is a good way to determine it, but not the only thing. Just like the fastest growth isn't necessarily the best growth.

Another issue is hibernation. Animals that hibernate tend to live longer when they hibernate as opposed to when they do not. This doesn't necessarily mean they didn't have an even better life because they didn't hibernate, but there are other things to determine.

I just don't see any evidence that a well-maintained tank or pond is any better or worse because it is warmer.