By some measures, it might be said that this was the week that AT&T finally cemented its hold over Time Warner, the media company it acquired last summer for around $81 billion. First, the telephone company prevailed against the Justice Department, whose appeal to block the deal was defeated in court a second time. Then, days later, two of Time Warner’s longtime media executives, Richard Plepler of HBO and David Levy of Turner, resigned.

The new AT&T overlords have emerged victorious, but their apparent win carries risks....