I've been noticing this particular aspect in this forum since I joined in February of this year. Many legendary members do not receive more merits than lower rank members, in my opinion. This led me to believe that they had attained the highest rank and didn't require merit to move up.
Yes, you cannot rank up further if you have a legendary rank, no matter whether you were a legendary before or got legendary after the merit system was introduced.
However, if you are a person who wants to join signature campaigns, then you will continuously look for merits as most signatures will require you to get a certain number of merits within 120 days and won't count your old merits.