That is why there are Merit Sources, to pump new merits into the System: https://bitcointalk.org/index.php?action=merit;stats=sources

Merit Sources are added (sometimes subtracted) every now and then to spur up the distribution process, and so is the merit allocated to each Merit Source adjusted over time.

Banned user’s sMerits are lost so as to say. Inactive users that decide not to come back, or even active users who don’t really play into the Merit System (specially with sMerits originated from the Initial sMerit airdrop) do add up to a considerable amount potentially, but as I said, sMerit Sources can theoretically make up for any lack of sMerit circulation, and have been adjusted in the past to keep the numbers on a certain level.

Edit: To answer your question, many of the large Merit Sources are known, be it by declaring it, or by other people detecting them (numbers have those things). Smaller newer Merit Souces may remain anonymous for a while longer. @Coin-1 does a good job of detecting most of them (see [TOP-200] The most generous users giving merits).