What you are witnessing now is the birth of a new martial art.
This annoys me.
MMA is not a martial art, because MMA is not a system. The term "martial art" implies systemization, it implies cogency of techniques and similarity of styles between academies.
MMA itself might not be a Martial Art on your definition, but new Hybrid-Systems are being born, sure, they are not big enough yet to spread in similiarties between academies, but as the sport continues to evolve i'm pretty sure some of the fighters/trainers will reach a point of influence where they will have created and spread their signature MMA Systems, altough it will probably take a few generations and some pioneers.