Since he won his world championship in Iceland 1972 Bobby expanded his focus past chess. He may have studied chess also but he didn't play publicly. [source Endgame by Frank Brady.] (Probably didn't play in secret either but that's neigh impossible to prove.)

If Mohammed Ali or any other famous boxer won a championship then quit training for a long time, would the champ be likely to retain his title?

Fischer refused the terms because we he knew he COULDN'T agree. He gave up his title in 1972/3 when he didn't stay in Chess shape.

He preferred to be a petulant diva who refused FIDE's match conditions to being the guy who sat there and took his lumps.