Except, instead of X merit in Y days, I would make it X merit in Y posts. That way, you're not penalized for low activity and you can't spam the forum hoping that some small fraction of your posts will get merit.
We have members on this forum who are more popular than others, and we have some that people don't like and receive less merits because of that and not because of the quality of their contributions. Someone like franky1 immediately pops to mind. I don't know how often he gets merited but I am sure some people have him on ignore, he is banned from posting in certain sub-forums, and some users might even hate his guts. Despite his dislike of certain Bitcoin features, I don't think he should lose his rank because people don't want to merit him due to the way he is.