Fischer had a two year break from tournament chess when he played Petrosian in March 1970 in USSR v Rest of World match, and won by 3-1. Right after that, Fischer won the World 5 min Championship by 19/22 points, losing only to Korchnoi (they split games). In April 1970, Fischer won at Rovinj/Zagreb with 13/17, and dominated the Palma Interzonal with 18½/23 points in November 1970, before going on to destroy Taimanov and Larsen 6-0, then Petrosian 6½–2½. Korchnoi had lost the 1970 Candidates Match to Petrosian by (−1=9). In the 1974 Candidates Match with Petrosian, Korchnoi led 3-1 with one draw when Petrosian withdrew (Korchnoi needed one more win).
Korchnoi is a great player, and he was nearly the equal of Karpov in the 1970s, ultimately edging out Petrosian in the early 70's. But Fischer was at a different level, and the two time offs from chess in the 1960s did nothing to dull his strength, so there is no reason to think he would have been dull in 1975. Neither Korchnoi nor Karpov would have been a match for him. But, he had nothing more to prove, and he was tired of FIDE disrespect.
The match would be very interesting, but Fischer should win