Your example with the camera module is exactly what happened to the FP3. They released the FP3+ which featured a better camera and users of the original model could upgrade by just buying this module.
However this is definetly not the focus of Fairphone as a company as too many or regular new modules would introduce new e-waste again.
But it’s 1 GB of backups. There seems to be no limit to how much you store, it’s offline after all and syncs peer to peer it doesnt cost them money if you store more stuff. However, I’d be interested to see how easy it is to store you backups on your own somewhere.