I think it'll be useless rank, there are already a good number of ranks and after merit system I think adding another rank will be not good option.
What is the motivation for a current legendary member to make quality posts then. I agree that most of the senior members are mature and understand the rules. But there are some who got lucky and achieved this rank because they had the activity at the time of merit system. Such members also need to be tested to maintain quality in posts on this forum.