You are right about Dilfer, although I think he made the Pro Bowl in '98, he was not that great they won the Super Bowl, or most other years. Brad Johnson was not that bad. Nick Foles is an awesome quarterback, if he's on a team with decent coaches. He could have been league MVP in 2013. Eli Manning is not an elite quarterback, but he played well enough a few times.
But you give the quarterback all credit, even if he's carried by the other 21 starters. In 2019, Garoppolo was okay at best, but he was surrounded by a great team.
Garoppolo and especially Shanahan are programmed to do terrible in the fourth quarter, if they had a lead.
I predicted in September 2019, that if the 49ers made the Super Bowl, they will lose after having a double digit lead in the 4th quarter.