On the elite level. 99.9% never make the elite level and fight for peanuts. If you don't make it in the NFL there's always canada. If you don't make it in the NBA there is the ABA and Europe. Both Hockey and baseball have farm systems. There is a better chance of making money in these sports than in boxing, and its also a way to a free education, something boxing cannot get you.
IMO its the fact that the neighborhood boxing gyms are GONE NOW. Nowhere for the kids to go and start boxing... plus the US amateur program is NOT what it used to be..... no where to develop the talent anymore
Frazier's far, far to weak for the NFL. Maybe he could have give the pro snooker circuit a go. That pays well.
Frazier was neither particularly big, nor strong, and he wasn't just "not fast" he was slow as a snail. He might well have gone to the NFL, but they would never have let him in. Likelihood is, Frazier wouldn't have been doing any other sport if he wasn't boxing. He was made for it. In fact he'd probably still go into boxing today given a similar upbringing, though he wouldn't be anywhere near as good. The sport simply wouldn't develop him in the same way.