Hero and Legendary already seem pompous enough names, which are difficult to surpass without going into the realm of the gods. Adding a prefix to Legendary seems to sell short on the pompous scale …
If meeting the current Activity required for a Legendary is between 2 and 3 years roughly, meeting 5K would take over 13 years, and 10K over 27 years. Not much of an incentive in real terms, and seemingly more relatable to becoming an aged relic on the forum, so as to say.
Since the Merit System was set in motion(*), 182 Legendary members (48 from scratch) have ranked-up needing Merits in the process, and 173 Heroes (71 from scratch). Not that many considering (besides all those pre-Merit System ranked members also existing).
There could be room for additional ranks, although I wouldn’t say that there is any seemingly good reason for it beyond elevating the ego of those that would meet the requirement, at most. Currently: