Well, this would be the first time, if he had played Karpov, that he would play someone younger than himself. His run to the championship in 1972 was astonishing. He literally crushed everyone. 6-0 vs Taimanov and Larsen and then 4 pt wins over former WC Petrosian and WC Spassky. Fischer had, as we now know, serious psychological problems. He had achieved what he wanted so much to achieve--the World Championship, breaking the Soviet monopoly. What really was there left to accomplish?

As for GOAT, that's difficult to say because the game changes and the total amount of knowledge has increased tremendously since the advent of computers. Fischer's play though, when analyzed by computers, is incredibly accurate. In My Great Predecessors: Fischer Volume IV by Garry Kasparov there are several passages that say things like "and the computer cannot improve on Fischer's play for the final 14 moves."

For me, the 5 greatest players of all time, and I'm not ranking them, are Capablanca, Botvinnik, Fischer, Kasparov and Carlsen. Of course, there are good arguments to include Morphy, Steinitz, Lasker, Alekhine, Karpov and perhaps Anand.