Just One More Thing Too Many.
Historically, the television production business model involved selling enough seasons of a show to a network that would broadcast it on a weekly basis (and cover your production costs) so that you could make hay by taking it to syndication where secondary networks would show it daily. Roughly speaking, this required about one hundred episodes or five series.
The twelve episodes of Fawlty Towers are often held up as the exception to this rule but listening to a programme about Columbo this week, I discovered that in thirty years of production they have only produced an output equivalent to three seasons-worth. Further proof that customers don't need diversity via product and line extensions but will remain loyal to something that meets their needs over and over again.