I would love to be more active here
I have conflicting answer to your question.
We do not need more merit sources if only the current merit sources would distribute merits effectively. I know they aren't paid for it but I am sure majority of the merit sources whose allocations are high doesn't exhaust their smerits within 30days. Some aren't regular and some doesn't care much about their status as merit source.
If all merit sources would be like the ones that create threads to issue merits, the ones that follow up the created threads, the ones that spray merits across every month and the one that offer post reviews via pm, there won't be need for more merit sources.
If I were theymos, I wouldn't make the merit status permanent or semi permanent, I would be rotating it for unserious merit sources, while those that spend all available smerits and even ask for more would remain and even have larger allocations.