I think it can be improved like this:
Brand new: Same
Newbie: Same
Jr Member: Same
Member: 10 but must get at least 1 merit for each 100 activity points to maintain the rank, if not, user will drop to Jr. Member
Full Member: 75 but must get 5 merits for each 200 activity points to maintain the rank, if not, user will drop to Member
Sr. Member: 150 but must but must get 10 for each 400 activity points to maintain the rank, if not, user will drop to Full Member
Hero Member: 450 but must get 50 for each 800 activity points to maintain the rank, if not, user will drop to Senior Member
Legendary: 750 but must get 100 for each 1000-1200 activity points to maintain the rank, if not, user will drop to Hero Member
The question is does the 21,045 sMerits being generated by merit sources enough to satisfy all the members from dropping their ranks? Now I don't the exact number of active members here in the forum but 21k won't cut it even if it send half of the sMerits sent to them it still not enough to guarantee all the members can receive sMerits. The merit system is designed to make post better not create a turmoil in the community in which makes members desperate on ranking up with the "Rolling Merit" idea you have you will only create merit selling more profitable for people who are actually doing it, as what have been discussed before they are doing their business elsewhere and they'll have a huge demand for their merits once this will be introduce.