Spassky himself said that he thought that Fischer would have defeated Karpov in 1975, but that Karpov would have probably won the next Match. Fischer had not played any competitive Chess for 3 years, and one reason for his demands (of which only one was refused) was that he wanted lots of games in the Championship so that he could play himself into form.
One must remember how badly Spassky was treated by the Soviet authorities after his loss to Fischer; this must have affected his Chess ability and concentration. Fischer refused to play for a relatively minor reason. It appears to me that he had already lost much of the desire to continue Chess as World Champion.
Nice lyric...what song?