And of course the "last ditch effort for another season or movie". The Stargate showrunners were stupidly bad when it came to this. (...) A bad movie leaves viewers annoyed at wasting a couple of hours. A TV series with a horrible conclusion leaves longtime viewers disgruntled and sometimes quite angry. But yes, a well-constructed TV show can provide many, many hours of great storytelling.
Yeah, except then the product (you!) has already been sold to the advertisers. And you've probably bought all but the last DVD/BluRay set, which means you'd have to be royally pissed and not just a little miffed to not buy the last one. They really don't have anything to lose that way, and it did get them Stargate: The Ark of Truth and Stargate: Continuum. It started with a movie becoming a TV series that ran for ten seasons with two spin-off series that ran for five and two seasons respectively plus the two movies I already mentioned so yeah... I doubt they think it was stupid, it seemed to work pretty well for them. There's Star Trek, who else has 17+ seasons and 3+ movies?