This led me to believe that they had attained the highest rank and didn't require merit to move up. They can contribute whenever they want or not, and even provide detailed answers to questions that deserve to be answered.
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.
What I’m curious about is that only a few legendary guys are always active in the forum and providing lasting solutions to questions posed in the forum. Others that are active only post to complete their signature posts. Does that imply that after getting to legendary rank, making posts or comments to be merited is not necessary again?
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.
As others have already suggested, OP's statement isn't true. I've noticed that legendary members still acquire a ton of merit, despite not practically needing it in order to rank up, but because they're worthy members of this community. I think there's even a thread here showing how merit has been distributed.
Although it may suck for newcomers, I didn't find it too hard to rank up. I was here before the introduction of the merit system and had managed to rank up to Sr Member. In approximately 2017 or 2018, the merit system was introduced. I a little after Bitcoin's crash, which took away all my interest in Bitcoin. I quit shortly after, not because of the merit system, but because I completely lost my faith in Bitcoin. Signature campaigns were much more popular back then and finding one wouldn't be that much of a hassle, but most of them were paying in terms of BTC and not USD. Thus, the rewards were minimal due to Bitcoin's value. At least in the campaigns I participated in.
I returned in early 2021, during the Covid-19 quarantine and after Bitcoin had skyrocketed once again. I didn't have any merit except the 250 I was distributed due to being a Sr Member. Within approximately 1.5 years, I earned more than 250 merits without acknowledging too much of Bitcoin's technicalities in order to further assist the community. I didn't have too much available time between university and a full-time job, but I managed to rank up by trying to make worthwhile discussions and posts and being patient enough.