Marciano isn't just swimming upstream, he's climbing up the Niagra Falls here. His forte was relentless pressure and stamina. Unfortunately that's kind of what Usyk does too, except for that he's bigger with next level skills. For what its worth I reckon Usyk would smoke Louis too.
Usyk wins for me Marciano's footwork wasn't the best and he's going up against a fighter with top 5 best footwork of all time in Heavyweight division. Usyk is also much more skillful than Marciano and definitely has the cardio to deal with Marciano's cardio which is also good. I'm not trying to be harsh but I think Usyk would make Marciano look quite pedestrian plus he's 6'3 with a 78 inch reach. I guess if Marciano could somehow get on the inside and use his power and strength he could maybe pull it off. But Usyk has dealt with much bigger fighters pressuring him and come out the victor. I just think the skill level gap, footwork, and height/reach advantage is too much in favour of Usyk. Usyk by decision for me although a TKO on cuts wouldn't be out of the question.
Prime Joe Louis beats Marciano let alone Usyk, Louis was ahead of his time but Usyk is still too skilled, Marciano wouldn't be able to handle it.
I think Usyk's foot work would be too slick for Rocky,and his handspeed too rapid for him to overcome.
It's all fun and games until Marciano goes to the body, Usyk hasn't ever been in a fight like one Marciano would give him, his stamina is pi*s in the wind compared to Marciano
Do consider that Usyk overcame weight differences that were working against him due to skill alone, whereas in this match, both weight and skill will now favor him. I don't see any way out for the Rock.
Only chance for Marciano is to somehow drag Usyk into a savage brawl and try to outlast him; but that is not very probable.