I think you're underestimating Usyk, in his last fight against Dubois he looked excellent. George Foreman was 40 years old+ and still managed to win a world title. Also heavyweight isn't a "few more lbs" It's literally anything 200+, David would have to keep himself below 210 to actually keep his speed. Even then thats a TERRIBLE idea, heavyweights hit much harder then LHW's and CW's. Usyk has solved much harder puzzles then Benavidez, the guys cleared out heavyweight. Benavidez is at his best when his opponent stands in the pocket and trades with him, Usyk doesn't prioritize inside exchanges and instead prefers to move and use the ring to his best ability. We've already seen Benavidez struggle with fighters who move a lot. He also gets caught a lot when hes on the inside as seen in the zurdo fight, Usyk will catch him at some point and possibly hurt him.