Fox doesn't have the same bench it did. There is no one with the credibility and charisma of Carlson left. When O'Reilly left they still had some talent.
The man was such a joke that it took Jon Stewart all of about 5 minutes to get him deleted from Crossfire.
That job has two requirement: the ability to look directly into the camera and lie, and the ability to keep the elderly afraid of non-white and non-binary people. If you can do that, congratulations, you can have that job.