The TV business has been shrinking for years, which is why media companies are trying to sell their cable networks. What now?

The occasion for this pre-mortem is, of course, the firing of Stephen Colbert, who announced two weeks ago that Paramount Global, CBS’ parent company, is canceling “The Late Show With Stephen Colbert.

As a deep-pocketed producer, David Ellison helped breathe new life into Paramount franchises including “Mission: Impossible,” ...