Lance doesn't look like he's ready to be a starter yet and Jimmy G is healthy and back where he normally is: somewhere just below the elite QBs of the league but better than a lot of starters. He's paid fairly for who he is and I think the track record of QBs being brought along slowly is way better than QBs who were thrown into the fire as rookies. With all the picks Shanahan and Lynch invested in getting Trey Lance, he will get his shot to be the starter eventually. If the team is winning without him right now, why rush him? Best-case scenario here is that we win with Jimmy and we ease Trey Lance in as the starter in 2022 or 2023 when he is ready and gives us the best chance to win.
