I'm actually in the minority in that I liked middle-of-the-series seasons 6 & 7 the best, though I also really enjoyed 1-5, and 8-9.

Season 10 was a noticeable decline in quality, 11 fell even further (the departure of Chin was part of it, but the writing was much worse too), and 12 was an abomination.

It should have ended after 9, though as has already been pointed out, people would be complaining today that it ended when it was still good, and should have continued. It probably did "earn" a 10th season by its season 9 quality, but in hindsight, I would have rathered it ended there.