Boxing is still more popular than MMA. Also the idiots who run the UFC are doing good job of ruining MMA....
Sure lets take the two biggest names in boxing out of the equation, two guys who spent decades building up their marketability, to make a point. Doesn't work that way.
MMA isn't dying by the way, it's still growing. It's just the momentum which has stopped. There was a massive surge 5 years ago with Lesnar among others, what they have now though is more younger fighters in the process of training with the hopes of making it and more gyms. I don't know if the hype will ever be as big again, but they wont die.