Coming from PMCO you shouldn't act like this is some new policy or surprise. You correctly note PMCO didn't have expiring RDM, but they did have account closure (which would impact RDM and LFM) after some period of inactivity.

Wayback machine has it from 2007 and several other years: If no mileage is deposited in your account for 18 consecutive months, your membership may be cancelled, and your miles may be forfeited.