Did you watch the James Toney vs Couture fight I linked above?
Toney beat Holyfield in a boxing match... Holyfield beat Tyson multiple times. Couture destroyed Toney. Not because Couture is a better boxer, but because boxers... ALL BOXERS... have no take down defense, no ability to check kicks, little or no wrestling background, no submission skills and no submission defense skills.
I have no doubt Tyson could have learned to fight in MMA... but he didn't. This video shows what happens when one combatant is trying to box, and the other knows how to change levels... execute a takedown... utilize a ground game and lock in a submission.
029 - Randy Couture Vs James Toney - UFC 118 - 28-08-2010 - Vídeo Dailymotion
You are literally telling one guy 'you can only throw punches' and telling the other guy "throw kicks, punches, use take downs, go for submissions, etc etc etc" - it's not a fair fight because they aren't using the same skill sets.