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.
the minimum amount of merits not spent, based on the merits put in circulation, at the moment is about 32432, i attempted to calculate it with my tool, so the number is going to be much higher than that.
You can see the script i used in here:
https://albertoit.github.io/Merit-Explorer-SQL/this doesn't keep in account merits from the starting airdrop and in particular merits which where never spent at all, but is easier to pull some number based on the merits sent at least.