I can think of one Don Majkowski, who lost his starting QB job for good after injuring his ankle early in the 1992 season. The sub who replaced him: Brett Favre.
Drew Bledsoe, Tommie Maddox. Lots of guys lose their jobs due to injury, all it takes is a backup that’s better than you. And it stinks, I feel bad for Smith who seems to be a pretty bright grounded guy that managed to survive a bad situation in SF. But that’s life, Kaepernick so far appears to be the better QB.