In this case, i do get your point... But what if somebody that used to be very active and a real asset of the forum took a leave of absence just before the merit system was introduced and wanted to come back right now? Wouldn't he/she be entitled to the same perks as the rest of us instead of being stripped from his/her merits just because he/she didn't log in for 5 years? There are plenty of cases of really helpfull people that just went away, but they used to be super helpfull. I'd still like those old accounts to be able to wear a sig, even if they haven't posted for years... And if they come back, they should still have their merits, their signature, their rank,...
Imho you are only partly right.
An old account posting useful stuff has probably received some merit already, even if the posts are ancient.
An old account coming back after 5 years may not be interested in signatures / earning (unless sold or hacked!), may probably be more interested in the actual discussions (which will probably get him merit fast).
So yes, you are somewhat right, demoting users would be "not nice". But it may solve more problems than it would create.
Maybe Theymos would start thinking at least on slowly decaying the airdropped merit (and related rank), and that's for everybody, no exceptions.