hmm i don't know...demolished? He wouldn't beat some of the best people who are more well-rounded, but I'm pretty sure he would be able to get up there with just his boxing, especially if he adapted it slightly to fight kicks and grapplers.
I think it would be very unlikely. They all would take him down. The very best of MMA fighters' boxing is at best, sloppy, compared to Mike Tyson in his prime or maybe even now.
Tyson would have never fought MMA during his prime because he made 100x more money & fame in Boxing. The only way he'll risk his legendary legacy to fight MMA now is if he was really strapped for cash. And then, we'd be hearing the excuses that it's the old & tired Tyson, just like Toney, Mercer, etc. who tried to make a buck in MMA because Boxing no longer wants to pay them.
Look at the Cheick Kongo vs. Pat Barry fight from last summer. That fight was settled almost completely by punches, in fact it was won by one solid punch at the end. Not saying its common, just saying it happens, and would happen more if someone like tyson entered the cage.
That's more due to DANA WHITE telling all the fighters that he's only going to book exciting fighters and not the ground and stall, boring ones. Early UFC's main attraction was the ground game of BJJ, but now, people are sick of that and wants to see more standup fighting ever since Maurice Smith, a kickboxer, won the title. Fighters are adjusting to satisfy Dana, who is trying to satisfy the demands of the paying PPV public. Dana won't flat out admit this, but it's pretty obvious. And if it were Tyson standing in front of them, it would be incredibly stupid for any MMA fight to try to box him.
Also, you're talking about a sport, boxing, versus another sport, a bit different than a style verse a style. Now if it was ancient pugilism versus someone learning judo and muay thai (or any combination most UFC fighters learn) who knows how pugilism would fare.
Well sport boxing since I specifically mentioned Tyson.
While ancient pugilism is not that big of a deal, IMO. Muay Thai is pretty old and it already encompasses almost all aspects of standup fighting. Muay Boran is certainly old, and much more brutal than Muay Thai. And the modern combat fighter should be more powerful and resilient compared to ancient ones due to the leaps & bounds of advancement in nutrition, sports medicine, etc.