Sly's enduring success broadly comes down to the Rocky and Rambo franchises
Each tapped into a perennially relatable human archetype:
* Rocky - the Underdog who wins
* Rambo - the Outsider who fights back injustice
Stallone was able to create these two characters so believably and credibly as he was both of those things, hence he COULD write Rocky as he himself was an underdog who wasn't given much hope of success, and he was also very much an outsider with no connections, a hangdog face and a funny name
The extra ingredient was he combined them with a macho exterior and powerful body, and so for the audience - mostly men - it was both relatable and aspirational
Then once he hit the gravy train he was savvy enough - aside from the odd detour - to keep giving the audiences what they wanted
All up he still comes across as earnest and likeable - unlike say Arnie - even though clearly at his peak he exhibited some pretty narcissistic tendencies, the outsider underdog who won and had a cool superhero physique to go with it
Well played Sly, a good life