Unfortunately the merit system will always have an element of chance in it, depending on who reads which threads and how many sMerits they have available at the time, which is why there are initiatives like the one I linked to before to try to identify and rectify any posts which are un- or under-merited.

Merit decay will have no effect on this though. Regardless of whether or not airdropped merit is decayed, there would be the same amount of sMerit available and the same amount of merit being circulated, and I wouldn't propose to introduce decay for sMerit. It's up to regular users not to hoard their sMerit and distribute it.