OP, are you familiar with the history of the merit system, i.e., when it was implemented, what the forum was like beforehand, and why Theymos created it?  It doesn't sound like it, but you'll probably get all the answers you need from the replies here and I suggest you read them all. 

My answer to the above is that there were tons of Legendary members prior to the merit system (before Jan. 2018), and they didn't have to earn merits to get there.  Any member who was below that rank on the day Theymos came down from the mountain and bestowed upon the shitposters, account farmers, and all the assorted reprobates his tablets of merit requirement, they immediately had to earn merits in order to get there.  If you were a Hero member the day before and were right about to rank up, you then had to earn 500 merits to get to Legendary. 

Obviously it was, and still is, much worse for newcomers to the forum.  But to your second statement above, anyone can contribute whatever they want, whenever they want.  That's not exclusive to Legendary members.  Nobody needs to rank up.  That's a requirement people put on themselves, usually because they want to maximize their campaign/bounty earnings.

Not sure what sections you frequent most, but there are plenty of Legendaries posting all the time.  Sure, many of them who've been members for years are less active, but that's to be expected.  People lose interest or have less time to participate in discussion forums all the time.  And once more, it isn't necessary for anyone to post, earn merits, or anything else on bitcointalk.  Members you see posting very rapidly or very prolifically are usually shitposters trying to hit their campaign's post quota for the week.  Again, those are usually the members with the least to say.