Its risky, plotting out a show with any form of arc-based narrative. No ones likely to throw all their toys out of the pram if NCIS or CSI or any other acronym-based television programme gets cancelled out of nowhere, because theyre procedurals, taking advantage of the formulaic structure of your basic network television show. Theyre built to be pretty much the same in season ten as they were in season one, thats how they function.
But when you build in any form of over-arching story to a show, you form a social contract with the viewer: youre saying, this will entertain/intrigue you right up until we choose to end it, at which point you will leave satisfied that the story has been told. It doesnt necessarily have to have the ending that the viewers want, but it should have the ending that the show, the characters and the story demand. It should make sense there should be closure for the viewer. It should feel right.
That doesnt always happen. For whatever reason, sometimes a show enters the final stretch and its lost everything that made it an important part of your life the cast has changed beyond recognition, or the shows been cancelled before it had a chance to finish organically, as intended. Sometimes the producers have lost sight of what made the show great. Sometimes the show hasnt delivered on its promise. Sometimes its just bad now.
For every Friends, finishing on top with a feelgood finale, theres a Rules Of Engagement, where its blindingly obvious that no one working on the show was remotely interested by the time it limped to the end. And just like Highlanders 2, 3 and 5 dont exist and never did; you can retcon your enjoyment of the show to leave off the offending final chapters. Hey, its your right as a fan.
Be warned: there are likely to be spoilers lurking ahead