IMO Trump has no real use for a chief-of-staff in the White House.
I heard Anthony Scaramucci (the little guy who was in the WH for a couple of days) say on TeeVee yesterday that Donaldo has his own way of doing things that involves establishing a "hub and spokes" system and that he needs people he trusts and who accept his personal judgment, judgment based on his own "feel" for situations.
I have seen this kind of methodology many times before in the world of sole owner entrepreneurial business. In that world egotism is king and the owner/wheeler dealer stands alone surrounded by underlings and consultants. For him they are nothing. They are expendable assets who exist only to serve his egocentric will and interests. They are there to be useful to him and can be disposed of whenever they are not. Trump operates exactly that way. Subordinates are disposable at will. Institutions mean nothing to such a man. He needs a secretary to run errands for him, not a chief-of-staff who will inevitably wish to be a "player." Anyone who takes the job is a fool.
In this context the case of the Trump announcement, a year in advance of his term's end, of a replacement for the CJCS, General Joseph Dunford USMC is interesting. Trump has announced that General Mark Milley, the present US Army Chief-Of-Staff, will succeed. The question is – why announce now? And why announce this now with a "footnote" to the effect that the "transfer" date will be announced at some future unspecified date? Milley is a loquacious, big, and energetic man who is reportedly quite good at the backslapping, locker room chit-chat that Trump is comfortable with. He undoubtedly has made a good impression on Trump in personal contacts and impression is all important in dealing with Trump.
OTOH Milley is really not like Trump. He is an Ivy League product of Princeton and Columbia Universities, is widely read in history, is personally as brave as a lion on the battlefield and has a record of working well within the institutions of the armed forces for systematic re-structuring of the Army. I will guess that the president doesn't really know much about Milley. IMO he will inevitably and quickly be displeased with Milley when he is CJCS.
So, why has Trump done this? My present theory is that DJT is displeased with Dunford and wishes to hold over his head the threat of quick dismissal. This is a close analogy of the way people like Trump operate in business where it is routine to undermine subordinates for the purpose of creating insecurity leading to prostrate submission to the throne. pl