Boxing is a part of MMA, but its only a part. Striking in MMA is boxing-influenced, but it is not boxing. There is a lot that that is different, from not being able to protect yourself with the smaller gloves to having to worry about kicks and takedowns. Everything changes.
With enough cross-training, a very good boxer should be able to become a very good MMA fighter. In theory. But that same theory should hold true for any practitioner of any major combat sport that makes up part of MMA - jiu jitsu, muay thai, wrestling, Sambo, etc. It just doesn't always carry over, though. Some guys can't overcome the instincts they've built in for their specific sport. And some just don't have the ability to adapt to different forms of fighting.
Ture but you can see more and more guys focusing on it. Rampage comes to mind.
I think if you could teach a good boxed to sprawl (ie take down deffence) and he was built in away he could keep separation with jabs, and at the end take into account the 4 oz gloves or what ever they use today a boxer could fuck people up. But not with out training in the other disciplines.
Thinking a MMA guy who has to spread his focus over Wrestling,Striking,JJ ... Ect. could beat a boxer that has focused his whole life on just boxing, in a boxing match seems far fetched to me.