At same time, the current 100+ merit sources have a distribution of 30.000 merits each month. This means from start a creation of at least 15.000 sMerits each month. And this amount, combined with all the other existent sMerits makes the situation asked by OP to be highly unrealistic.
This is very simple for the OP to understand, and now with these, I think the OP will understand more that there is no way that merits will be scarce in the forum as long as the merits sources still exist, even if all the 100+ merits sources have distributed the merits that were given to them all. There are still other forum members that have merits that merit sources gave, so they will be the ones to continue giving out merits, and the merits system will continue like that.
However OP, remember that one of the reasons why merits will not be scarce is that the Smerits you see do not get used if we keep them instead we should give them out so that other low rank people will rank up to higher ranks, and you can see that some people coming to the forum now have more merits than even their activity, so if these people will still use the Smerits they have to other people again But I can say that if the number of merits source we have reduces, then from there the number of merits on the forum will reduce, but we cannot say it will be scarce at all; instead, it will be low.