Niners drafted Lance. Jimmy started and got to the NFCC game. Lance broke his finger that year, which derailed his development.
Lance was the starter the next year, but broke his ankle in game two or three. The Niners couldn’t trade Jimmy because Jimmy got some surgery at the dumbest time possible. He got hurt later and Purdy came in. Purdy was fantastic, so the Niners moved on.
If Zach was the QB, the Niners were playing him year one. If he wasn’t ready (or the Niners thought he wasn’t ready), they wouldn’t have taken him. Lance was a special case, with elite physical tools but very limited experience. Zach had the experience. There wasn’t going to be any “wait and see.”