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.
Yep, I completely agree with you. I'm against any kind of de-ranking or account "erosion".
The part of my post you quoted was meant as a tweak to an idea that both DdmrDdmr and I had. The idea is that if you're trying to discourage low-quality posting then it's probably sufficient to just enable/disable a member's signature based on some criteria.
Assuming a good fraction of bad posts are from signature campaigns and that campaign managers won't pay if your signature is disabled, then having your signature temporarily disable itself when you've gone too long without receiving merit is a very simple and effective way to make people think twice before posting.
Like you said, there are users that struggle to get merit for reasons that may not be fair (although franky1 seems to be doing okay for himself [1]) and like ETFbitcoin said, there are boards with low merit circulation that put some users at a disadvantage, but I think those two things can be (mostly) worked around by setting the criteria to be forgiving enough to accommodate nearly all quality posters.
[1]
https://ninjastic.space/user/franky1?merits