Maybe I'm being an alarmist, but let go from their positions or resigned, either idea strikes me as scary.
HOW it happened is unsettling to start. But one way or another, all things must end. They've served a LONG time and despite my ignorance of these individuals of yet, I think it'd be easy to gamble their time under Trump would have been short given their tenures' accomplishments. Who ends up replacing them is more important to look out for, in my young-inexperienced opinion.
This sort of thing definitely results in (greatly) reduced capacity to project and preserve American interest, reduced stature in the international community, and attendant material resources to train or otherwise correct these losses. It possibly signals to Trump that he should stop being a dick. Or something. But at the same time, looking at that equation... I still can't help but side with the managerial staff in their decision. I straight up don't know what the right course is.