While this is obviously very far from feasible, what _is_ possible is a return to servicability for the two parts that most commonly fail: batteries and screens. The rest of the phone is usually fine unless subjected to serious physical battering. The legendarily reliable Nokia 3210 is your landmark here.

Note that in a modern phone, the screen strength usually is achieved by bonding it to the case, so in practice "replace screen" turns into "take mainboard out of case and place in new case".

Modularity doesn't really help at all with the constant drive to replace working phones with newer, better ones. But there should be an effective secondhand market selling them to the Third world - and there is! No shortage of websites offering people money for their old phones.

Long term, we have to wait for the Moore's law slowdown, and a rise in Chinese manufacturing wages and other costs, before a local western repair industry becomes viable again.

(Apart from all the other issues, phonebloks would turn "Android fragmentation" into a far more extreme version; you can't guarantee that your peripheral will even be there next time you turn the phone on).