I think this post makes several valid points. Firstly, in the link above which is the official UFC site they define MMA.You know, another possible reason (assuming your assumption is correct) is that more likely the people attracted to MMA will just do MMA and have no background in any other style. So it's possible that in a few years time there will be no contenders with a back ground in judo or bjj either. Just a mixture of moves from differing styles taught in a way that enhances their cage fighting taught purely in a MMA school.
Below lists what I assume is the leading backgrounds for UFC contenders.
UFC® Martial Arts Styles - Fighters Train in Multiple Disciplines
So the premise in the OP is flawed before we even start. UFC is not a competition between two competitors from two individual styles. It is a competition between two competitors who have each trained in multiple different styles to achieve a level of competence to fight in ring according to the rules of the UFC.Mixed Martial Arts is the fusion of two or more styles of fighting. Every UFC fighter trains in various systems to create the perfect blend of skills to compete in The Octagon™.
The second point is that potential fighters now have a choice. They can go off to a school that teaches an individual style such as TKD or Judo or Boxing and become proficient in that style then look for another type of training to complement their initial choice. So from TKD or Boxing they might select Judo or BJJ and from Judo they might select Karate or Boxing. Or, they might go straight to an MMA gym to get a grounding in multiple disciplines right from day one. This is what jezr is saying above, but I think it is already happening and in the future you will see fewer and fewer fighters who could be identified by a particular style of training, if any at all. MMA will just be a generic term for a style of fighting that encompasses striking, grappling and ground.
To my mind the Gracies did an absolutely brilliant job of promoting their brand, firstly by developing a competition where they could use their style to dominate, then to promote their brand from their success.
In UFC 1 there were fighters that could be identified as practitioners of a particular style but those days are long gone. Now you need to train in multiple disciplines to compete in a competition designed for fighters who have trained in multiple disciplines.
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