You can't change the past but you can change the future. Theymos could return every account to it's basic rank but that would cause a big chaos because there were a lot of established users. Imagine, when you see everyone with only one newbie rank with limited possibilities, it's hard to understand who was who and it's especially hard to understand for newly registered people. So still, this action was pretty fair. The reason why they implemented Merit was to lessen the spam attack from people who were creating new accounts. While you stop attack from new people, you can slowly prevent spam from current users and finally you get normal situation by "not burning any bridge".
Pretty much. While I think some of them really doesn't care about forum ranks, It would be quite unfair to make everyone start from scratch(newbie rank) even though most of them deserve the high-ranks because a lot of them are actually real early adopters that are actually genuinely interested in Bitcoin.
Sure, the airdrop surely rewarded the people who probably didn't deserve the merit airdrop, but it's the right move in the long-term success of the forum.