Eventually we will reach peak phone
PCs are replaced less often these days because, IMHO, there are few really major improvements in performance these days.
10 year old PCs can still handle an average user's workload.
Phones are still seeing leaps and bounds in performance, although not as startling as in the past, so the reason to update a phone is still there eventually.
In my limited experience one of the biggest issues with older phones is lack of memory. While daily or weekly updates of the software seem to leave the old updates lying around you are going to run out of space eventually. One of the many nice things we could have would be new releases of Apps which include all the old patches which could then be deleted.
[Thinks - if you are prepared to risk losing all your data could you just uninstall and reinstall all your Apps from time to time, or with locked down ones disable them and revert to factory then enable again to get a later version? Or would you just get all the updates in one go?]