They damaged themselves ... already ... Don had nothing to do with it.
I agree that the damage done, was done by FoxNews, to itself. But going forward, Trump could have blacklisted FoxNews, which, as long as he is popular, hurts FoxNews ratings.
In reality, there is something for both sides if Trump agrees to appear on FoxNews product - another business calculation that changes, depending on how FoxNews leads the discussions.