> It's nearly impossible for even a star athlete to succeed at more than one sport

I don't think this means as much as you're putting weight into.

I think the fact that somebody like MJ or Tim Tebow could even get a try-out in a different profession sports really speaks to top level talent being fungible. Like just imagine somebody practices being a Doctor for 20+ years and then gets an interview to be a World Cup Referee. Sure, they might not succeed at the other sport but the fact they can still do it is I think proof of sports fungibility.

It's not that top athlete's aren't well above average in a sport that they didn't dedicate their life to, it's that without that dedication they don't have a chance of being a top athlete in that sport that disincentivizes potential athletes from training in multiple sports.