I think one of the themes that is running through this excellent question is:. Can a fighter remain in his prime longer by moving up in weight?

Yes and no. (Come on now this is one of those times where I don't take a slightly extreme position 😂)

Yes because clearly at the lighter weights the fighter will reach the top of the curve (of his prime fighting abilities) in general a lot earlier than a heavy weight would, and in my opinion that is largely because smaller lighter people just move faster in general (of course Muhammad Ali is just one of several pieces of evidence to the contrary), but the average lightweight or welterweight simply has faster hand speed than the average cruiserweight or heavyweight for example.
AND SINCE IT IS PRETTY COMMONLY AGREED UPON THAT SPEED AND REACTION TIME ARE THE FIRST THINGS TO GO WHEN YOU ARE BEYOND YOUR PRIME, it would seem somewhat logical that moving up to a higher weight class may be able to (AND OF COURSE THIS IS CASE BY CASE), may be able to buy you a bit more time in your career....

However plenty of evidence to the contrary would go something like this:. Even if moving up gets you opponents who don't have quite the hand speed as your lower weight division, the question becomes: can you take the harder punch from the bigger guys in the higher weight classes? And as a corollary to that exact same question, do you have the power to hurt anybody at all in the higher weight class with the bigger guys? (Roberto Duran is an example of someone who clearly DID have it).

Lots of questions and lots of exceptions to what seems like it should be an easier question to answer but once you start peeling back the layers you realize it is a real conundrum

Then you got a guy's overall health. if he was able to get through his division relatively unscathed then you would think he would have enough left in the tank and not too much wear and tear to be able to succeed in a higher weight division and buy himself some more time/remain competitive for longer.

I think there are so many factors involved that in the end it is a case-by-case basis, but in general if I have to decide all things being equal, I guess most people agree that a heavyweight for example has a much longer window, as power is the last thing to go, as we have heard time and again. So I favor an affirmative answer.