Ad hoc replacement is still a policy - just an unwritten, expensive one.
Devices get bought at different times, from whatever budget was available, at whatever spec seemed reasonable that quarter. Personal laptops end up alongside company-issued ones with no stated boundary. Patch and configuration standards drift device by device, so two people doing the same job are running meaningfully different setups without anyone deciding that on purpose.
Nobody sat down and chose this. It's the default that happens when replacement is triggered only by failure, and standardization is discussed only after something has already gone wrong.
Endpoint lifecycle treats the fleet as a decision: what the standard configuration is per role, what triggers a refresh besides breakage, where BYOD fits, and who is accountable for keeping the fleet at that standard - not just for fixing the device in front of them.