Hmm let's see:
Basketball GOAT: MJ, highest career pay of his generation
Golf GOAT: Tiger Woods, highest career pay of his generation
Tennis GOAT: Nadal (disputed), highest career pay of his generation
UFC GOAT pre-Conor: GSP, highest career pay pre-Conor
The whole point of professional athletics is you get paid based on your performance. Therefore the athlete with the highest career pay is either GOAT or in GOAT conversation.
https://www.businessinsider.com/highest-paid-nfl-players-2015-11
Highest career pay in NFL: Peyton Manning, in GOAT conversation.
Give me an example of a pro sport where an athlete who gets paid significantly more than everyone else in his career is not at least in GOAT conversation. I'll wait.