as you mentioned in your third point, boxing will draw talent more readily than MMA at this point because of the allure of dollar signs,
also acess to training is another factor- lets face it, while a boxing gym might not exactly be cheap, add on seperate grappling training now as well and you are increasing the budget by quite a lot. Also figure in that managers and such are going to have to lay out more money to train a full-time mma fighter and the returns are less for them as there is less of a "peice of the pie" at the end of the day.
in short- there is less money for "managers" and less to draw fighters.
wrestling, in all honesty I'm not as up on because the last time i actually paid attention and wasn't drunk (kimono wannalaya and that hot blonde making out on top of the ecw arena doesn't count) was when elizabeth was still fuckable.
I would imagine though that the same goes for them (as someone else mentioned) if you are in the wwf or a major league.
I think the statement "never" is not really the intent obviously. I think he means more that will it be common place for people who have made names for themselves in boxing and professional wrestling to switch to MMA.
my answer will be that people will follow the money.
Crossover athletes are rare in any sport. I predict that they will follow the same pattern with MMA. When you train your whole life to do one thing well...you're probably going to keep doing that thing well instead of trying something you'll only be average at.