That's why I prefer that the series stay intact, rather than turning it into a movie franchise. If they can keep the standard of writing and production as it currently is, I will take the series over a movie every time. But remember, most actors, no matter how popular a show is, get very bored doing the same thing over and over. It happens to every 'hit show'. They also fear being 'typecast' in their roles as well and start looking to get out of their 'contracts'. That's when concessions are made with actors to cut their "workloads". The Producers would rather have 'some of Actor X' instead of nothing at all. The movies are a way to have their cake and eat it too.

The question is "when do you turn it into a film franchise"? They run the risk of series fatigue. You do it too soon and they lose money because the series is still profitable. You overstay your welcome and suddenly the fans that were once there are gone, making the franchise profitable an uphill battle. Case in point....The X-Files. I was a HUGE fan of that show, but they f*cked it all up with how they handled Duchovny's role in the series. They also stayed too long as a series and the movies have suffered because of it. Fight the Future was more or less an extended episode to bride the season, whereas I Want to Believe was trying to revive a series. If and when a new X-Files movie comes along, nobody is going to care. Had they made a conscious effort to segue to full-on franchise mode around Season 6, they could have made a franchise that was sustainable.