Bizarre that this is even debated anymore in all honesty.
The best fighter in MMA currently is an ex low level boxer. His two best wins are also ex low levels boxers.
The question about ‘how does boxing translate to MMA’ was answered years ago, and it seems to be pretty well, up there with other legit combat sports, even when the boxer in question of pretty poor. The only interesting question now is how does elite boxing translate, and the logical conclusion is that they would likely do very well.
Even looking at the intangibles that make good fighters, they have them all by the absolute bucket load. Conditioned like the best wrestlers, with elite level punch resistance, hitting harder than anyone on the roster and coming from a sport where quitting is taboo enough to end a career. Obviously they’d do well.