I joined the forum in 2014, had two large periods of inactivity (approximately 3 years away in 2018) while I wasn't active in the first place when I signed up. The merit requirement hadn't taken place back then and managed to rank up till Sr. Member somewhere in 2016 or 2017 if I remember correctly. I re-entered the forum in January 2021, having the 250 base merit due to the Sr. Member rank.
So far, within approximately a year and a half, I've gained exactly 200 merit, requiring only 50 merit for Hero. I agree, ranking up is not an easy purpose, while merit isn't the perfect system by any means, it has successfully got rid of spammers who were unable to rank up and join signature campaigns.
Yes, there are a lot of older members who may not have earned their forum rank due to the quality of their posts and their contributions to the forum, but they was lucky to have become members before the merits system started and thanks to that they got a higher forum rank, while new members have to work much harder for higher forum ranks.
Because of this, it could be said that the merit system is not fair to the younger members, who joined this forum much later, but on the other hand, thanks to the merits, we were able to separate the quality members from the spammers on this forum, as you said.
No system is obviously perfect, but it is obvious that merits have benefited this forum.