Someone in the industry told me that they don’t renew shows after a couple of years because they would have to pay more for them. Basically, they’re operating like a sports team that relies on rookie contacts to maintain profitability.
My understanding is that television staff unions all include jumps in pay after certain numbers of seasons. That, coupled with declining viewership after 3-4 seasons (most shows seem to peak in season two or three, means that the show gets more expensive per view.
They might be able to get around this if they could design a show that has a constantly rotating cast and crew, so that by season 4 they don't have any original people left.
The Crown changes the main characters’ actors every two seasons.