I was envisaging a doom loop scenario. Mac has a disaster (because he ignored the policy experts and then no-one dared/was allowed to tell him it was a problem till it was to late), he then sacks a competent civilian and brings in another ex-military sycophant. Then there is another disaster, so he does the same. As his cabinet gets worse and worse it becomes more and more military heavy. His habit of everyone calling him "Sir" leaks out. He keeps wearing a military uniform. Then Luce and Willoughby spectacularly balls up the response to Dien Bien Phu and that turns into a foreign policy disaster. Each on it's own is not serious enough, but cumulatively they push things over the edge.I think it’s still pretty implausible. There’s no serious thought to ban people from certain backgrounds running for office OTL even despite current circumstances. Short of Mac starting a WWIII/a nuclear war or trying to reintroduce slavery it just doesn’t seem likely. Worst case scenario is Mac get impeached and everyone moves on with a wary ness towards a military member running in the future.
But then I'm not convinced by BNC's last analysis of MacArthur and still have quite a dim view of him. Given our author's views on Mac we're not going to see a complete catastrophe, so I agree things will never get that bad in this TL.