People really have to see Wilt and Russell during their peak, it's a shame that there weren't more archived games from that era.
Sure, the average player in today's game is likely more athletic, or even more skilled, than players in those era, but the cream of the crop were plenty athletic back then and now.
Just look at it this way, Russell and Wilt battled it out, Wilt even held his own during the later part of his career against a young Kareem, and an old Kareem was all-star level all the way to the late 80s, in the age of Ewing and Olajuwon, and Olajuwon battled a young Shaq and often had an upper hand.
Going by that chain of logic, Russell and Wilt would still be effective in today's game. They will probably not average 50 ppg and 25rpg because the game has slowed down, with more sophisticated offensive and defensive schemes, but Russell will still be a 15rpg/3 or 4 block/ 4 or 5 assist guy. He'd be like a Joakim Noah + prime Wallace on steroids kind of guy, someone who you can anchor your entire defense around while directing traffic on offense. Wilt will probably still be a 30/15/3 guy (Shaq level), or at least a 25/12/3 guy. That is of course, factoring in today's training and fitness regimen.
Of course, pure speculation, but from what I saw, their games will still be effective, if not dominant, in today's NBA.