On the Croatian local board, we actually have a situation where almost all members have reached the desired forum ranks and no one needs new merits.

I think you always need merits, even if it's legendary.
The merits are the recognition that what the person says/does is useful for the readers of this community, whether that person is legendary or not.
In addition, merits continue to be useful for users to be able to participate in campaigns. Therefore, for me the merits are much more than just ranking up in the forum.