The basic deflation scheme of merits (halve each distribution) is only one of reasons for potential stall.
The changes of activity or inactivity of users is another reason. Ones can earn lots of merits and have lots of smerits to send but what if that user turn into inactive (abandon the forum, inactively for long period, months or years; or ends with death). Merit pool will naturally dry out without merit sources.

Merit sources serve another role. They play as source of merits for good posters, not all users. Theymos does not want to do a second merit airdrop to all users, including many shitposters.