Really the flaw with all the "move on from Jimmy G but get a cheap vet" theories is the same....
If you do it you're tanking the year. And that's fine, but you have to go in knowing that, and if you were you shouldn't have done all the things you did to this point, or said the things you did.
Tanking a year to develop your rookie QB is fine, but if you do it after telling your fanbase (and owner, and players) for months that you think you're a title contender, you've just made yourself a massive mess.
There is no FA QB who could start and be as good as Jimmy (when healthy), in fact the FAs are all just awful (probably the best one is.... Nick Mullens who they had last year).
So either you keep Jimmy, or you trade for a QB (likely not as good considering he doesn't know your system) so you now are giving up picks to get a QB, to replace your current QB, just to add a bit of cap space next year.
To me there are two paths that make sense for SF:
1. Keep Jimmy, hope he rebuilds value and you can trade him after the year for a decent return.
2. Trade Jimmy for anything you can get now, accept that you're finishing last in your division, and that the fans will be angry and hope your rookie can win the lockerroom and look good by year end.
Edit- the key is there is not really a reasonable middle ground, there isn't a reasonable way to replace Jimmy with a QB for less money that doesn't drop the ceiling of your 2021-22 team.