Cunningham was fast and had an 82" reach how exactly is rocky going to land on Fury with his stunted arms ? He wouldn't even land on Fury I actually liked the rest of your post
Slightly younger Wlad, Fury, Povetkin, Joshua... all can move, cover a lot of ground in a short time and cover it laterally.
Fury was also laying his chin out there, goofing off. Once he got serious, the trouble escalated quickly for Steve.
The problem is we compare Rocky's footwork with LHW fighters and the above guys footwork with SHW fighters. I don't think Rocky would be slow of foot in today's HW division. His reach isn't really an issue due to his style, leaping hooks and inside attacks don't require long arms. Any fighter that lets him inside will struggle . Just because Wlad mastered the art of the clinch doesn't mean every SHW can nullify to the same degree. He's obviously got his work cut out, that's why I don't think he'd bother. He'd be a LHW/CW and maybe move up for 1 or 2 fights.
I like your theory. Rocky would dominate at the weights lower than superheavy and challenge a great big guy in a super fight when the money and circumstances suited him. And I agree a special fighter that size is special enough to win selective key fights at that weight class. If for example Beterbiev proves to be a once in a generation phenom (and he might not) there is no saying (If he does prove it) he can't take a mediocre SHW champ as a one time deal is there? I get that. But the thing that keeps nagging me is these giants "with skill" only exist because of the Spinks experiment. Remember wrestling before the 1980s? Big guys couldn't fly through the air off the top rope. All that started with steroids. Before the spinks experience big guys were always horribly disadvantaged in boxing. You take the modern advances away from today and Rocky, Dempsey, Louis (as they were) wouldn't even need to put weight on. They would cream all of them.