Peak athleticism hasn't improved in any noticeable way. The difference between then and now is the learned skill level improving because of what has been learned from the past, rule changes that forced offensive and defensive scheme innovation, and a huge expansion in the overall depth of talent. Yeah, the average and below players of that era probably wouldn't have jobs now. But the top guys would learn to shoot 3's and play help defense. MJ was the best athlete of his era (I don't care what combine stats some idiot comes out with, MJ was built to play basketball. Every physical advantage you want, he had. 6'6", long arms, huge hands, incredible first step quickness and speed, leaping ability to rival anyone ever, strong enough to finish going up against anyone, still unparalleled agility and body control, especially in air). Claiming Demar Derozan could do 1/2 of the things Jordan could do is ludicrous and I haven't gotten into how much more skilled he was or how much better a defender.
Dream and Chuck could both create and played face to the basket enough to know they'd adapt just fine. Only thing that could stop Shaq in the post is the double team, and he was great at getting deep enough without the ball to make it impossible. A new Shaq would be a game changer even today. Especially if the new version weren't a lazy SOB and showed up every game and stayed in shape and practiced.