The rebuild will have to come regardless of Purdy's contract though. We've been in "win now mode" for the past 5 seasons, and next year will be the last one of that (failed) push.
So, from 2026 on, neither Brock nor any other QB is gonna have the same weapons we've had from 2019 to 2023 because our playmakers will be too old to produce on an All Pro level, so they will need to be replaced.
You then add to that the big holes this team has had but were able to overcome off the strength of those playmakers, specially the OL, and you have a 2-3 year project regardless.
So at the end of the day the question is if the Niners want Purdy to lead the rebuild and hope he continues to improve to make others around him better. Or discard him after the 2025 season and try their luck with rookie QBs -which in today's NFL is very hard to get one worth a fukk, getting Brock to click with this scheme and players was a miracle in itself- and spend on the OL and other positions like they did in 2019-2023.
Now, we also gotta take into account that Jimmy G got a dumb contract (I remember he was the highest paid QB for a while, lol) from us at the time, because management decided to keep building with him as the starter. So, as
@King Kreole has stated,
overpaying Purdy does not automatically mean he'll get no help and no more playmakers to make this scheme work.
It's gonna be a challenge either way because we've seen this team not be good even with all its weapons when they don't have a suitable QB for Shanahan's particular needs. With Brock, the team knows he can produce much better than a Jimmy G (in the regular season and playoffs the numbers between them are very telling if you are smart enough to not just focus on W-L) so it's almost a given they will cave in, give him waay too much money and figure out how to "cheat" the cap to still get high level players around him.
Still, the 2026 season will probably be a really bad one so who knows if even Shanahan stays after it

how the Niners do the year before, in 2025, will probably shape the franchise for the next 5 years tbh.