First, the 9ers are unlikely to simply release a QB that has trade value -- they are waiting for a good team to lose their starting QB and be desperate, so they'll part with a few high draft picks. The Browns wanted Jimmy, but they traded all their first-round picks for Watson.
Second, the 9ers have to be a little nervous about Trey Lance, and they're going to keep Jimmy in reserve, just in case Lance falters or gets injured. The 9ers are loaded and don't want to waste this year if Lance isn't the guy.
Finally, in the unlikely event the 9ers did release Jimmy, there's no way he'd be willing to go to a team that is not playoff bound. For a guy who's played in 2 out of the last 3 conference championship games, signing with a team that's in serious rebuild mode is not going to happen.