Andy, I think this is a special idea again, and although I like special idea's, I also wonder if it would make a better forum.
As far as I can see it only reduces the quantity of posts in the end, but it doesn't reduce the quantity of nonsense posts at the moment of posting.
Also, both of your idea's don't do anything about the
quality of posts. As long as there is no Lieutenant Commander Data (assuming you know a bit of Star Trek) no computer can do that.
It seems you want to equalize the amount of posts of each user. Well, in that case you should post a bit more often. Maybe then "off topic life" would be more interesting.
Edit:
I forgot to say: Since your system would delete all old posts, it makes it impossible to read back what's been said half a year ago. And it probably would happen many times that a post is deleted by accident, because a user forgot to "stick" it.
Edit 2: (post below) I think the size of the database is not really the problem.