I agree, milking shows usually ruins them. On the other hand, if they are great shows that last a long time you can gain a loyal audience. This seems to be how HBO has found so much success- Sopranos (never lost quality, even by the end), Curb, the Wire, etc. Although, GOT got milked out terribly. I think the hard part is just deciding which shows to drag on- and being based on a story that already has an ending helps guard against plot degradation.