Why do people keep parroting this without actually explaining why it would happen? MMA isn't going to go to a boxing model anytime soon for a variety of reasons, but why do you think a boxing model where the promoters have to pay a significantly bigger slice of the pie to fighters would necessarily disadvantage fighters who don't draw? Are fighters who aren't at the top level and don't draw doing particularly well in MMA at the moment in terms of pay? Why would they get a smaller slice of the revenue in the event that promoters have to pay out more (I don't think they'd get much more, but there's no reason to expect they'd get less from what I can tell).
Also, with regards to promotions re-investing in boxing, boxing has an international amateur structure that MMA doesn't even come close to. I've been hearing for 15 years that the UFC is pouring tons of money into amateur MMA, but the rate of improvement in that amateur infrastructure even as the UFC profits have exploded is pretty meagre (or, in some areas, utterly non-existent).