A few observations
Well I just think if Nadal had 16 slams (or even if he surpassed the record), I doubt he would make more than Federer's total fortune. I'm sure he would make a lot regardless, but I just don't see him making nearly as much as Federer. Federer's success comes from his records as well as the Swiss identity which is associated with luxury. But all I'm saying is opportunities can come from cultural background and appearance as well. In the case of Federer, this is a small case (most come from his records and association with Switzerland). But in the case of Sharapova, it's a huge one.
Well that won't happen, so why discuss it? I associate Switzerland with army knives, cows, mountains, skiers, chocolate and banks. Not really with luxury.
Sharapova isn't born American, but not only does she make more than Serena Williams in cosmetics, modeling, etc. She also is surpassing those two sisters in gear and equipment as well (take a look at her lucrative contract with Nike). She signed an 8 year 70 million contract with them in 2010. The previous year she won 0 slams while Serena had won 2 slams. Ironic, isn't it?
So in the case of Williams/Sharapova it's a lot extreme.
Williams is ugly, is on tour part time and has a bad temper. Pova is good looking when seen from a distance and is a real pro: working hard day in day out on court and off.
So yeah, if Nadal did become the GOAT, I think racist and prejudice stereotypes about Spanish people would come to play in his earnings when it comes to international endorsers and promoters. Federer's reputation would have also been affected somewhat if he was born German or Austrian. Given that these were not associated with "neutral countries" the quality and quantity of endorsements and promotions could have varied.
Come on... Rafa is not Mexican for God's sake.