The cease and desist letter from his lawyers accuses Bannon of breaching a non disclosure agreement and also of libel. If Trump proceeds along this course, we are going to get a "dirty laundry" slander lawsuit the likes of which America has not heretofore seen - ever. Bannon will defend himself by saying all this dirt on the WH is true and told in the public interest. Trump will try and hush it up and argue that Bannon is a lying pos. It's likely that some of Bannon's dirt will stick to Trump. In any event, the lawsuit would be front page news - along with the Mueller investigation - throughout 2018 and into 2019.
This is another blow to the already-frail credibility of the presidency and the only potential winner is Bannon, whose 15 minutes will drag on another few months. Bannon is the ultimate emperor with no clothes. He appears to be a dim-witted, disruptive and egomaniac bully whose empire is built on Breitbart - a news source of low repute and little credibility - and the Mercers' $$$$$$$. At a certain point, the Mercers are likely to drop him and cut a private deal with the Koch's to both "share" the GOP and its politicians. And Breitbart won't live forever on the gullibility of its core followers.