I could see decaying airdropped merit, though I'm not convinced of it currently. If it was done, it may be best to do something like this:
- Your current activity score as of is recorded, which I'll call EPOCH_ACTIVITY.
- Your merit score is defined as max(0, AIRDROPPED_MERIT-(CURRENT_ACTIVITY-EPOCH_ACTIVITY)/7) + EARNED_MERIT.
If I'm reading the formula correctly, active users would lose 1 airdropped merit per week (actually, they'd lose 2 airdropped merits with every activity increase, which is on a biweekly basis.) If it's something that is important to you and the community, that seems like a fair way of implementing it.
Those who are active and contributing to the forum shouldn't have trouble keeping their merit score up, and those who are here to shill and spam will be able to see themselves being slowly demoted. An airdropped Legendary who earns no merit will be demoted to a newbie in a little over 19 years. If you can't learn to earn merit in 19 years, I guess you should be demoted to a newbie.
I like that the formula doesn't affect inactive users. It does seem like a very fair compromise.