You had me until you brought up Wrestling. I don't think you have many MMA fans who have/will crossover into wrestling. I think a lot of boxing fans turned to MMA when the boxing scene died down. If you're into MMA how could you not be into boxing? But, I could understand boxing fans who never got into MMA and don't like it. MMA even though it's a new sport and doesn't have near the level of athletes that boxing does it's just a lot more exciting to follow. Boxing has just not evolved with the times. That's why there just isn't a lot of boxing talk going on anywhere.
-Disgruntled pro wresting fans have turned to MMA in droves, thanks mostly to TUF Season 1 being aired after WWE RAW, and then the Brock phenomenon. Also, MMA gives them what wrestling used to give them and now fails to deliver.
-A staggering number of people who describe themselves as MMA fans were pro wrestling fans at some point (hi rob!), although many have given up on it (see above). Many studies have backed this up, and no matter what he says in public, Dana White knows it better than anyone. Just look at the way he promotes his product. Also, as UFC buyrates have risen, wrestling buyrates have fallen at almost identical percentages. Draw your own conclusions.
-Don't even get me started on Japan, where prowres/MMA are interchangable, promoted by the same people, and covered as if they are the same entity. Different culture, but they simply aren't snobbish about prowres and recoginize the obvious similarities. (To be fair, wrestling has always been viewed as more of a respected art form in Japan, as opposed to being considered low brow "trash" entertainment in North America)
-MMA fans generally like boxing.
-Boxing fans generally dislike MMA. Alot has to do with the media contrived "war" that doesn't really exist. Mostly it has to do with old boxing writers and TV media feeling threatened by MMA and ripping it to shreds in columns/rants/diatribes. This trickles down to fans, who view MMA as less technical, and buy into the "barfight" stereotype.
-Boxing/prowres have very little crossover, with the exception of latinos (particularly mexicans). Ironically, it's the latino population that has largely kept boxing relevent, it's white/blacks/younger demos where the interest has slipped over the last 10-15 years or so.
I agree that MMA fans rarely cross over to wretling. It's the other way around. Your typical MMA fan has moved on.