I noticed that so many users so far haven't spent any of their sMerit - usually Bitcoint/altcoin discussion spammers who probably don't even know that Merit system exists. Probably these guys who analyze stats can give exact numbers.
I don't like your idea. Every user have right to hold their sMerit and not spend. I don't think that we need to force users to spend their sMerit. I don't think that decay of initial Merit is needed. It wouldn't be fair for users who aren't active on Bitcointalk but earned high rank in past. And they would lose it only because they didn't spent their sMerit.