(*) Back when rank used to be post-count-only, and things were adjusted to be based on "activity" (in June of 2013), many posters were all like "Why change this? FFS, I liked how it was before!". Then, when an additional term was introduced (in January of 2018), again, many posters were all like "Why change this? FFS, I liked how it was before!". So, it's inevitable, IMO, that any rank-hardening adjustment made at this point, even one that manages to "thread the needle" just so and long-term improve the forum's signal-to-noise ratio without causing too many casualties in that pursuit, will, at the time of its proposal or implementation, be met with some amount of, wait for it... "Why change this? FFS, I liked how it was before!" 
No offense but we have already an ongoing issue about merit circle jerker that farming merits on spam thread, WO and local merit thread.
Your suggestion is pro merit jerker and will remove the natural poster that doesn’t have any good knowledge on the board which merit is heavily distributed. Your suggestion is feasible if merit source is available on every thread of the forum and no bias on merit sharing, hence everyone should have an allocation of sMerit per month in able to cover the merit distribution on the forum just like the reddit upvote and downvote power for everyone to gather karma.
You are trying to build a contribution group like github and not a forum by adding a stricter rules that requires merit just to keep your rank. It’s bitcointalk, I believe any form of discussion related to Bitcoin given that it’s not a spam is already a contribution here CMIIW.