Good point. Banning all pre-2018 users would solve all concerns about the fairness (or lack thereof) of the airdropped merits.
The best thing to do is to ban those users who have no earned merits until now. When they get banned, quality posts goes high. In addition, those newbies who failed to earn a single merit for 2-4 can be considered also in a list of banned users because they failed to achieve a single merit.
It's horrible idea to ban user who's been inactive before merit system is introduced. If theymos plan to do this, i'm sure few people will start meriting currently inactive user, but have made some contribution in past rather than meriting recent good post/thread.
He probably didn't understand that this was subtle humor ... Apparently, sig 'roobet requires a really large number of publications from the participants.