While I believe that BBing and MMA are compeltely different goals, they do have one thing in common... too many young kids walking around as dellusional wannabes.
Just as 10 years ago when every 15 year old idiot started doing pushups after school and wearing his t-shirts a size too small, was walking around with elbows jutted to the sides, hoping someone would notice, Now we have the same type of idiots walking around in their 'Tapout' or 'Affliction' shirts, or sporting the too-tight wife-beater tank tops and mouthing off about how they're an MMA fighter to anyone within earshot(not saying that they train, but that they're a 'fighter').
I studied (and even taught!) karate for a long time when I was younger, but I never felt the need to ramble on about it OR try to present myself as a 'fighter' (I wasn't, I never had a professional fight in my life!). Even after BBing for YEARS (16 years now), I always would tell people that I 'train', not that I was a 'Bodybuilder' (although now that I've competed and have a bunch of trophies, I've changed my tune a little bit -lol).
People need some sense of identity, and something like BBing, or MMA, that seems to have a preconceived reputation attached to it (being tough?) is the simplest way for a young kid to achieve some sense of one.
Even with the popularity of MMA, I doubt Bodybuilding will die simply because the people who are serious about it, who embrace it as part of their lifestyle, do not get involved for any fleeting glory or sense of self. It becomes something they do, and usually will continue for many years to come. I think MMA if more of a short term thing, especially when you consider the wear on the body, I can't imagine too many competitors having any considerable longevity the way a BBer might (and to be honest, most BBers don't have much longevity either!)