I think it depends on exactly the rules.
Are we talking about magically plucking RSM from a decade and a half ago to play a game? He would struggle as balls are different, and he's optimised for the game back, not the game now.
Are we supposing RSM were born at such a time he's in his prime now, and developed from a youth in the modern game? There's be no reason why he wouldn't be one of the top players. His style of game might be... different.... but there's no reason why he woudln't still be at the very top levels of the game. Footwork, feel, physicallity, game knowledge and metality. The fundamental requirements are the same now as they were back then, and he has all of those.
Perhaps the most interesting, would be if we plucked say, an 18yo RSM out from his time, and let him play the modern game for a few years, then see how he does. I think in this case, he'd still be a top player, but probably not winning any major tournaments with strong competition.
It may be an unpopular opinion, but modern players are almost certainly, on average, objectively better players (in pretty much any sport). Not because they are necessarily more talented, but because they have the luxury of being from a time where we know more, and do things better.
We know now how to: train better, eat better, prep for a game better, recover better. And we've had more time to try more ideas, and seen more ideas tried by those before, those amount to some fairly compelling advantages over a sporting lifetime.