There's really nothing Shanny or the team could have done. He sat his rookie year because he was incredibly raw and we had a solid QB coming off a SB appearance, no one had a problem with that. He was given the QB1 position in Year 2 over that same QB coming off a Championship Game appearance, this was supposed to be his prove it year but he got a season ending injury very early and the eventual starter played very well and never lost a game. Year 3 he was given the chance to showcase why he should be the starter in offseason and preseason but hasn't looked better than QB3.
Shanny/Lynch's mistake was in drafting him and giving up all those picks, but they gave him his shots. If he was as good as Trevor Lawrence then he would have started rookie year, and if he was as good as Mahomes he would be starting over Brock this year.