What is "far longer" when still on rookie deals? What's the extra upside comparatively when you have those players locked into cheap money, fully under your control for a set period of time? Especially when a huge number of players don't actually blossom until their third or fourth year?
Just keep rotating people in the locker room across the entire roster? What message does that send - especially as they already do it a ton with the end of the roster (practice squad and UDFAs)?
Stability isn't just balancing in the pocket when someone has their arm around your ankle - it's also confidence that you'll have a job tomorrow and work to keep it. While yes, there are those where hanging the Sword of Damocles over their heads is a better motivator, for most it's not and is actually the opposite - dispiriting and demotivating.
I agree with the general idea that many players will take two to three years to develop. For instance the jury was totally out on Jeff Allen after year 1, but he got it together in year 2 and now he's projected to be a franchise quarterback. JE11 took a few years to find his role as a punt returner and special teamer then grew into the clutch WR we now know him to be.
The down side is you are using up a roster spot and a bunch of coaching bandwidth and related resources hoping they will develop, with no way of knowing if they ever will become a franchise level player, or a starter, or a backup, or a bust.
IMO the QB position is so important you should take many bites of many apples, keep tasting till you find one you like. The down side of that is those are some costly apples. Think of Carson Wentz, Kirk Cousins, Sam Darnold, Mitch Trubisky, etc.
I don't think BB puts the same level of value on QB as many/most do. If he did, it would have been us trading up to #3, not SF. He values the potential contribution of every resource, and spending that much draft capital on one player seems to be something he resists, unless he sees a superb value.
Curran's idea is that there is no way BB goes into the year with just Cam and Stid. That's not too high a bar to clear, just pick a fourth round guy and use him for a camp arm and you clear that bar.
I'm sure BB sees Cam for what he is, just a placeholder till he can find someone better, yet he won't over-pay for something better since the team overall talent level still needs improving. Basically Cam is the Vinny Testeverde of his era. Bill would rather tread water and improve the overall team player by player rather than pay what SF paid to move into #3 to get one key player.