Season 8 is where I think it really lost it for good.
In season 8 Benton left the show (he took another job with better hours to allow him to be with his son more) and Green died. By that time the show had lost Jeanie, Doug, Carol, Mark and Peter. It was just too much to overcome.
Personally I didn't care much for the Pratt character, so we not only lose some of the main characters from the original cast they replace them with a character (that to me) was annoying.
The show limped along for a few more year but to me it is when these characters died that it really became unrecoverable (although I'm sure to others the show had lost its luster long before that).
For some reason, to me, Mark Green was kind of the anchor of the show, it gave it its compass.