That would have pretty drastic effects. Looking over a specific subset of profiles (those that have either merited or have been merited historically), I make it we’d get:
The first figure represents the number of profiles that meet the criteria. For example, 111 Legendries have earned over 60 merits in the last 6 months (meeting the "must get 10 every month on average per rolling 6 months" criteria), making it 6,93% of the Legendary profiles I’ve studied, and thus 93,07% of Legendries would be de-ranked to Full Member in your proposal (or whatever other rank).

The figures are really an overkill, and would really destroy pretty much the ranking system. If the merit requirements were to be high-ish (the above are not that high, but not many people make it), it could even give room to collude to not merit someone in order to de-rank them (a bit remote, but possible).

Note: In addition, there are other profiles I have not accounted for: those that have neither earned nor sent a single merit so far.  I'm also simplifying a bit, and omitting the added complexity of changing-conditions once ranked-up (i.e. a Full Member profile just turned Sr. Member with 30 earned merits in the last 6 months may be automatically demoted for not having the 42 required over the last 6 months to keep the Sr. Membership).

No point really. They would either spend them without giving it much though, or open a thread on Meta complaining for being de-ranked when they don’t care about the Merit System.