If you didn't stop posting you may had enough activity and merit to rank up already. If you use forum in read-only mode your activity stays same.
Some people may stop posting because not only activity is used again when merit/smerit started in 2018, spammers and shit poster stop posting by that time because they may be alts or because they are not having merits which helps this forum to filter away spammers and many shit posters. He may continue to post, but according to his post history, if he did not change and increase his posting quality after the implementation of merit/smerit, he can only have more activity, not merit and he will stay being a full member. I will not be surprised that if there is no merit/smerit implementation, this user would be a legendary by now but on a shit posting forum, but that was changed with the implementation of merit/smerit.
Now that things have changed with the ranking system, only those being active, posting, and contributing to the forum's growth can have the opportunity to grow in rank, of which i also agrees with you that it's the best measure in fighting against spam and shitpostings, if a user remains on the forum and yet never know why he needs both activities and Merits for growth indicates he never read through the Meta section here to have discover the pinned threads on Merits and new ranks requirements, such people are the lazy type that enjoys a disadvantage but never care to know why, except for those that does not have the intention of staying here.