Tbf, FP4 will still have replacement parts available for years to come. Not as good as having a (potentially indefinite) upgrade path, but FP4 isn't obsolete.

I don't think this will ever happen, it's an anomaly unique to the PC market. The first time Fairphone did this with the FP3, allowing users to upgrade their camera, there was some minor OS workaround added to make this work, because the hardware in phones just isn't set up to handle this.

IIRC I believe the workaround involved trying to get the installed camera to do something that the old one was not capable of doing, and if that operation failed it would spin up the driver for the upgraded camera. PCs use shared memory and a bunch of other standardised stuff I'm unfamiliar with to simply ask a device what it is, but phones can't do that for their internals (for the most part)