I agree with you, but it is such a tough thing to manage. I speak on this as someone who has built forums with 100K+ users and millions of posts. Incentivized activity is almost required to get new forums off the ground and are tricky to change as they are often motivation for the core base.
Some short posts will be fluff to pad stats, but others will be legitimate and helpful.
The issue as I see it is users selling accounts and the ease of pulling it off. Once the accounts have a monetary value the difficulty in moderating quality grows exponentially. It's too much for any staff and must become a job of everyone in the community.
I like the idea of the merit system, but am concerned that it could just make vested accounts more valuable and further incentivize account sales. My suggestion would be to make it more difficult to sale and transition an account. Not foolproof by any means, but more hurdles to change email addresses, SMS verification (with random re-verification) or new IP login challenges could help. The counter to that is those steps all run the risk of annoying the user base and likely add to the operating costs.
In spite of the issues, I admire how this community has grown. Administrative guidance is much more difficult than most realize.