Shows should end. 9 Seasons is the most any 1 Hour drama should after last, and then it has to be a general show that's premise isn't based on a problem that requires a resolution. Somethingt like the X-files, that could be about really anything works better for the long haul. Even then, though, it was pushing it and should have cashed it in with DD wanted to leave the show. It got really stale after that.
It works better if you have Arcs for long run shows. A plotline is brought up and then resolved, and a new one starts. Course if it's a show like SG-1 it's the plotline over and over again just with different names. You know, the DBZ syndrome. But if I ever found myself producing a 1 hour drama. I'd go for one arc a season, and maybe 5-7 seasons at most. I'd also try to figure out how it ended first,and work backwards from that. That way I could make sure that small stuff pointed towards that through out show. That's how I'm writing my books, and it seems to be working pretty good.