I disagree. Boxing has suffered because:
1. It's an inferior product to MMA.
2. Good fighters refuse to fight each other.
Now, item two you could partially blame on promoters but if the UFC maintained match making control, they'd get around that.
One of the problems I have is that fighters are signing away their likenesses FOREVER. Even if they retire or wash out of the UFC, they are still owned and can't make money with their likeness.
On top of that, the UFC inks a deal with Reebok for their fight gear shitting off revenue streams for fighter sponsors on their clothing/trunks during fights.
The UFC has gone from small PPVs with a title sponsor of tiny companies like Condom Depot to record setting PPVs with sponsors like Bud Light, Reebok and Harley Davidson. And despite that, fighters are not making significantly more money. They're getting screwed.