I don't know why you say there "would be" a legitimate beef; I think there clearly is one. As someone who flew numerous flights on CO during the 5+ years of 50% EQM earnings, my EQMs for that period are far less than the actual BIS miles I flew. I think it's unfair that MP members get this increase in lifetime miles while many CO flyers don't even get credit for all their BIS miles.
Indeed, and a UA flyer who flew a partner during that time received EQMs but no BIS. The one-time adjustment adjusts all lifetime miles to be based on EQM regardless of PMCO or PMUA - PMCO already used this method.