How can we solve the problem of unmerged RFCs after their FCP?

Agreed, but it's not just that rfc. There's also #3161 and #3641 (the latter even having a comment asking for timeline for merging). I think that this is an issue that should not occur at all.

(I didn't include the mitigation enforcement RFC because it is comparatively recent.)

The main problem I have with this is the finality of FCPs (final comment periods) not being entirely representative. After a final comment period, where the community has the time to raise objections, the RFC should be merged as a decision has already been made and all concerns already being answered. So after the final comment period, IMO the RFC should be merged as soon as possible (immediately?)