Limiting the posts would cause new users to lose momentum. A lot of them might lose steam after joining and give up. That would be risky. Also, because a large proportion would give up, this would filter users. We'd end up with a larger proportion of the type of user persistent enough to tolerate this. I don't know what that sort of person looks like.

I'll add the idea to the pile, but I can't really sell it if those things aren't addressed.