John Kennedy pulled the plug on Laos the year before with his ridiculous neutrality agreement. The upshot was North Vietnam flooding the South, especially the border regions, with militants who could now operate from cross border sanctuaries in Laos itself and in Cambodia.
Of course Kennedy and his (mis)advisors blamed Diem for the growing chaos, and decided they had to pull the plug on Diem as well. We all know how that turned out.
There wouldn’t have been any Vietnam War, IMHO, if Nixon had been elected President in 1960. Instead, there would be a free and prosperous South Vietnam.
I have read that when some US officials broached the subject of removing Diem with some South Vietnamese generals they were horrified by the idea. They hated Diem, but couldn’t think of anyone else who could hold the country together.