Not so long ago, Rupert Murdoch’s decision to sell the crown jewels of his media company, 21st Century Fox, looked like an acknowledgment that the sun was setting on his reign. Now, as Disney and Comcast prepare to do battle over those coveted assets, Mr. Murdoch looks set to go out in a blaze of glory.

On Tuesday, a U.S. federal judge will determine how glorious that blaze can be. The judge is scheduled to rule on AT&T’s acquisition of Time Warner. If the deal is approved outright, it will be a green light to Comcast to make...