To me, a forum where tens of thousands of people are active posting very regularly like in here, should also be having an influx of new users that are able to contribute that is akin to its active userbase. That would be at least a few hundreds of new users becoming legendary every year. If we check
https://bpip.org/Report?r=mostmerit there's only one legendary user in the top 1000 with a registration date of 2021 and 14 users from 2020. So in a little more than 3 years only 15 users reached the maximum rank. Also note that the bottom of the top 1000 have only a tiny bit merit above the required 1k to become legendary so I wouldn't guess that there's many more besides that.
To become a Legendary w/ a 2021 registration date by now would mean posting just about every day (or at least 14 times per 2 weeks) since then, along with accumulating an average of ~1 merit every day... No easy feat really. It requires a lot of commitment to going beyond shitposting, which is a step most accounts here aren't willing to take. They are here because its "easy money"... once you have to put effort into it its no longer "easy" anymore. So they will never get beyond Sr. Member or Hero and they don't care.
I think it took me at least 4 years to get to Legendary based on the speed of my posting, which was intermittent. I don't think merits would have been a factor but who knows.