MMA has had a window of 10 years now to absolutely smother boxing and completely take over, and I think they've failed miserably. A lot of that has to do with dana white and his cronies being slime balls who let their athletes cheat and commit crimes so long as the fight on the PPV card then punish them afterwards.
The original UFC (no weight classes, one-day tournament with fighters having multiple fights) was the best possible product a fight fan could wish for. Since it became regulated, when it came back it tried to hang it's hat on the artistry that goes into MMA and attracted a solid niche fan base. They should have stuck to pleasing the niche.
Since trying to go WWE model with "drama" and trash-talk, as well as trying to break into the mainstream, the UFC has alienated the purists and all the casual fans who came in within the last 10 years are already bored with it.
Boxing will always be king.