This isn't actually true. Montana was let go after the 1992 season (he played in 1993 in KC). He missed all of 1991 and all but one game of 1992 with injuries. During that time, Young played in 11 games in 1991 and 16 games in 1992, and here were his numbers:
1991: 64.5%, 2,517 yds, 17 td, 8 int, 101.8 rating
1992: 66.7%, 3,465 yds, 25 td, 7 int, 107.0 rating
In 1992, Young led the NFL in the following categories: comp %, TD passes, TD%, INT%, yards per attempt, adjusted yards per attempt, passer rating, net yards per attempt, adjusted net yards per attempt, and pro-football-reference's AV (approximate value). In other words, in 1992, Young was one of the very best players in football. Montana was coming off 2 massively injury-plagued seasons and was 36 years old. Young was 31, in his prime, and was already established as the best QB in the game at the time.
Not really anything like the TB/JG situation.