There would of course be locker room issues with Fields/Caleb. Even the scant comparable history shows that for sure occurs (GB wasn't all sunshines and rainbows twice, nor was Aikman/Walsh. I'm not sure about KC or San Diego, but if you told me there was I wouldn't be shocked.)
But (sample size alert) there also isn't any indication/case of such a shared dev approach delaying or failing a guy. And while some guys like Manning overcome awful rookie campaigns and some like Herbert or Stroud wow from the start, cases like David Carr have most agreeing you can definitely ruin a guy. If you have the option not to, that's probably a reasonable approach that should be in consideration. Options better than not in most cases.
With Fields having aparently no market there isn't that much purported downside to holding. You destroy his value from a 5th to nothing? But if Williams would benefit from a red shirt year, that value is immeasurable. Or it's totally unnecessary and he could be Stroud day 1.
I think you can definitely weigh the risk/reward upside against the risk/reward of rushing your next shot at a franchise QB and the meager day 3 trade comp.
ETA - or even if a Williams is a Manning who works past rookie struggles, there's value in a higher floor 2024 and competing still - main point being there's a strong argument that rookie QB learning curves may be an unnecessary thing whereas they're sometimes viewed like necessary dev time)