Interesting idea, but I think this could backfire pretty badly.
You're basically taxing people for posting, which seems backwards for a discussion forum. Good contributors who help newbies or engage in active discussions would get punished just for participating. Meanwhile, people will probably just focus even more on merit farming instead of actually contributing useful content.

Also, merit distribution is already uneven across different board sections. Someone posting helpful stuff in the altcoin section is going to have a much harder time than someone in Bitcoin Discussion.
Maybe instead of penalizing every post, we could look at post quality over time periods? Like if someone's merit-to-post ratio tanks for a month straight, then apply some kind of warning or temporary rank adjustment.

Just my 2 sats - I think we want to encourage participation, not make people scared to post.