Generally the writers that were core start to leave, or they run out of ideas.
You could see that the Star Trek producers were probably correct to cap off the series at 7 years, and they could have probably milked it further with TNG and Deep Space and Voyager, but they would have jumped the shark into bad scripts and mundane stories. Aside from that the over-over-over abundance of supernatural / vampire what have you series is really bizarre to me. The rabid interest that supposedly rational people have in spook shows is strange. Science fiction makes more sense, supernatural stories > not so much.
The rabid interest that supposedly rational people have in spook shows is strange. Science fiction makes more sense, supernatural stories > not so much.
I disagree, considering that most science fiction programming has very little do with science or "rational" exploration of ideas or concepts and much more to do with space opera stuff and assorted goofiness. In the end, it comes down to investment in characters and good stories. I watch Supernatural because I enjoy spending time with those characters, not because I particularly care about the show's iteration of supernatural creatures, just as I watch Orphan Black because I'm having fun with it, and not because I'm sold on the science behind the shenanigans.