Even if the top rugby/mma guys could be top nfl/boxing guys, no one is going to pay them top dollar right away. They'd have to spend a lot of time proving themselves first at a much reduced income. These athletes have a limited amount of time at the top of their profession and it's likely the risk of being a flop isn't worth the years they may have to spend to equal or exceed their previous earnings. I'd guess a lot of low level rugby/mma types attempt to switch to more lucrative sports when they realize they're not going to be upper echelon guys in their initial field, but they're not famous, so no one really cares. Many sprinters and distance runners switch to soccer once it's clear they're no longer/never going to be in the top levels of track athletes, but no one pays much attention except in rare cases.