Instead of implementing a deranking mechanism with "carry," why not simplify the whole idea and base the rank directly on the ratio of merits to posts?
The higher your merit/post ratio, the higher your rank.

The need to maintain a high share of useful contributions will only apply until a member reaches the highest rank. Once you’ve achieved the top rank, it means you’ve already proven yourself as a respected contributor to the forum. At that stage, you should be allowed some “elder’s grumbling” and casual posting without fear of losing your status.

This would give a very clear purpose to reaching the highest rank: it’s not just about patience or consistency, but about maintaining a strong balance of quality to quantity. After that milestone, the forum recognizes your legacy and contribution, and you can enjoy more freedom in how you post.