Maybe it will need some time to push out shitposters, but the point is, shitposters have no chance to be merited by writing ennoying one-liners and so all shitposters will never rank up. This is a big advantage for quality posters, because the members will bei divided into such, who aren't interested in a serious discussion and those, who bring quality on the forum.
I agree that some accounts never will stop shitposting, it would be far from reality if we think the Merit system stops all shitposts. But in a way it is very sucessful even if you remind you can detect alts by farming Merit. This is a great achievement so far.