I don't think so, DaveF, because you forgot about inactive users. Your suggested requirements should be combined with activity points.
Thinking about it how about "rolling merit"
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You need:
Brand new: Same
Newbie: Same
Jr Member: Same
Member: 10 but must get at least 1 every month or you drop to Jr. Member
Full Member: 75 but must get 5 every month on average per rolling 6 months or you drop to Member
Sr. Member: 150 but must but must get 7 every month on average per rolling 6 months or you drop to Full Memeber
Hero Member: 450 but must get 8 every month on average per rolling 6 months or you drop to Full Memeber
Legendary: 750 but must get 10 every month on average per rolling 6 months or you drop to Full Memeber
-Dave
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
This will only make the merit thing more complicated. You know getting merit isn't an easy task as people are not generous in giving merits even to many good posts and dropping the rank level will make it more difficult to rank up.
Those who receive merit gets smerit too. They should spend it too.
This way merit will be more distributed and the deserving one will rank up quickly.