Aside from the merits generated a month from merit sources, the Smerit gotten from other members who receive merits still generates a lot of smerit when they are being distributed. Just as you have also explained, if everyone is distributing their merit and not holding back, I believe that alone can go a long way compared to the amount that is being distributed by merit sources every now and then.
I think before making this post he was not aware of merit sources and based on his understanding about how the merits system works, he thought at some point users are going to run out of merits.
I just want to buttress that if there are no merit sources, the sMerits generated by members would not be enough to keep the merit system running, it's certainly going to be exhausted. For example, when A merits B post with 10 merits, B also receives 5 sMerits, B give out 5 merits to C and who receives 5 merits and 2 sMerits in that order as shown below.
A (10 sMerits) ==> B (10 merits, 5 sMerits)
B (5 sMerits) ==> C (5 merits, 2 sMerits)
C (2 sMerits) ==> D (2 merits, 1 sMerit)
E (1 sMerit) ==> F (1 merit, 0.5 sMerit)
But the 33940 sMerits from 109 merit sources each month ensure that users don't run out of merits and sMerits.