However, we can't deny that pay per post model is successful one to attract a large user base of quality posters.
Of course I can deny that! When I joined Bitcointalk, I didn't know people rent out their signature, I only found out much later.
I can guess that many of quality posters today came to this crypto community encouraged by this model especially after implementing the merit system. As a proof, compare bitcointalk to other forums in quality posts after implementing the merit system which has a primary goal to reduce spam and it's successful.
You're using the wrong "proof". The Merit system was created to reduce the massive spam, which was (largely) caused by people posting as much as possible to rank up as many accounts as possible. Forums not monetized by users don't have this problem.