MMA as we know it today is a relatively new sport. It's really quite remarkable how quickly it has risen to the national (and international) stage.
The biggest sign that MMA has arrived is the UFC's deal with FOX. What was once only available on pay-per-view is now available over broadcast television. It's not just TV, however. I've seen a large number of MMA gyms sprout up in my neighborhood. You can also see MMA's emergence through the wide availability of MMA-branded clothing such as TapouT and the numerous video games on the market.
MMA has taken off like a runaway freight train. I believe it has overtaken boxing as the premier spectator combat sport. However, I may be biased, as I was never too big a boxing fan to begin with, and I never got to see the greatest boxers fight live (Ali, Tyson, etc.)
Because of its insane popularity, I think it's fair to ask:
Will MMA one day overtake all of the different martial arts that it is comprised of?
MMA is now a part of our history and culture, but not really a part of our identity (unless you are directly involved in it). Since it is not specifically associated with one country, I don't think it could replace the martial arts that are (China and Kung Fu, Korea and Taekwondo, Russia and Sambo, etc.)
MMA is first and foremost about competition. It does contain the attributes of other martial arts such as self-discipline and self-defense, but I think of those more as byproducts rather than being the primary focus. I think someone with purely self-defense in mind would train in something like Krav Maga rather than MMA.
Simply put, there are martial arts that specialize in certain things that MMA cannot cover.
It's only now that we are witnessing the new generation of mixed martial artists—those that train strictly in MMA from a young age. There really hasn't been enough time for there to be "MMA masters," but then again, maybe there never will be because the very definition of the sport itself is always changing. It's still really too early to tell if all the different forms of martial arts will one day meld into MMA, but I think the early answer is "no."