This is worst idea ever 
Only result from this would be increase of spam and worthless junk posts.
There is not enough discussions to pay for that, and I would reduce number of posts from 30 to 20 or 25 any slightly increase payrate.
It's not the worst idea trust me, if you remember Mixero campaign on Bitcointalk, then you will understand that this idea was working back then and it was a good one. Though, the amount paid per post on that forum was way high than on this forum for the posts but none of the members were making spam posts, in fact they were putting their best efforts to make those extra posts.
I didn't said that it would be compulsory to make 40, 50, or 60 posts, I just said that maximum paid posts should be increased from current 30 to 40, 50 or possibly 60. The minimum posts still should be around 20 posts or whatever a user can post but the maximum paid posts should be increased.
In initial days when signatures shifted on this forum then many people were thinking that it would be hard to make those 30 posts here on weekly basis but now many members make more than 30 posts a week and I believe that those posts doesn't fall into the category of spamming.
If signature campaigns make the average weekly posts 30, you will find that most members will make 20 to 40 posts. If the same campaign increases it to 50, you will find that the average has shifted to 30 to 60. the higher number of paid posts will force more to post, but it is difficult to do weekly 50 High quality posts, many of your posts will turn into a spam.
25 posts as a maximum is a good limit, and I don’t know why 30 is the average here.
Even if a manager makes the maximum weekly paid posts to 50 I believe a few members will manage to make that number of posts without spamming. I believe the maximum number of paid posts should get increased but the minimum should remain the same.
However, once the forum gets some good traffic and more exposure on search engines then most of the services could increase the rate of the posts and may reduce the number of max paid posts to 40.