TheNegation

No, mma is not in it's infancy. It didn't start with UFC.
TKD has had half a century to prove itself and has largely failed to do so.
I did TKD for two years. As a fighting style, it is innaffective. If it was useful in MMA, it would be used, but it isn't. Try me, I love a good debate, and I have trained in both Muay thai and TKD.
But you can't prove TKD works in MMA as a style, as awhole, because it hasn't.

Edit: And it does have more to do with the way you train than the art you train in.