I'd say there will always be enough merits even if some people are selfish and are keeping their smerits with them and aren't distributing them. As you said, there is a difference between merit sources and merit hoarders, and those who are hoarding merits can do that for as long as they want and that wouldn't have much of an effect on the circulation of merits since merit sources are given a quota each month that they need to distribute to worthy members.

It's not like merits have a limited supply like Bitcoin and those hoarding merits are eventually locking a percentage of the total supply. Let them hoard and use them whenever or wherever they want. Besides, theymos has already said something about hoarders: