The Niners were 1-10 in 2017. Jimmy started the last 5 games and they went 5-0.
Next season Jimmy G is hurt and they go 4-12.
Next season he's healthy and they go to the SB.
Next season he only starts 6 games with a 3-3 record and they finish 6-10.
Next season he's healthy and they make it to the championship game.
It always looked pretty cut and dry to me. Yeah Mullins, Beathard and Hoyer aren't that good but the Niners were picking at the top of the draft when Jimmy was hurt and the bottom of the first round when he was healthy. I'm not confident that some second rate backup can come in and just run Shanny's scheme and be successful in it. The most difficult position to fill is a good QB. You don't throw that away to ensure you can keep playmakers and defensive standouts. You keep the QB and then rotate the rest of the players around him. Young and talented WRs, RBs, DEs, LBs etc can be had all over every draft but talented QBs are few and far between.