Why not keep using them as...phones?
Snark aside, why are the entirely functional devices obsolete? It's because the growing demands of the endless software bloat, web bloat, feature bloat. New wireless technologies and better protocols, sure, but I've been using software for 35 years and the software contribution to this mess really gets me down.
Part of the reason why Android phones specifically are not supported for very long is because the baseband and modem firmwares from Qualcomm only receive official support and updates for about 2 years.
For everyone? I mean it doesn’t seem to apply to Apple, need it apply to Google or Samsung?
Apple only uses Qualcomm chips as modems. Almost everyone else uses Qualcomm chips as main SoCs.
Now, could hardware vendors tell Qualcomm to go pound sand and run their own support for old SoCs? Yes they could. Do they want to? Hell no, supporting old devices doesn't make any money.
My assumption is that Apple has a better contract with Qualcomm, being their biggest customer (for now, until they completely move over to their custom modems). Apple probably also has been abstracting the firmware from the start inside iOS, while Android didn't until project treble.
Samsung & Pixel are now offering 7 years of updates for flagships, so it would seem it's no longer a hardware/support limitation and purely a financial decision by other android manufacturers, and by Samsung for their non S-series of phones.
TL;DR OEMs are deliberately choosing to not support their devices, not due to any limitations anymore (thanks to project treble).
The screen broke on my S24 but I'd still like to use the compute, ram and storage.
samsung phones can be plugged into an external display and used like a computer right?
Because a person doesn't need to carry 3 phones, but they could be 2 security cameras and 1 phone instead?