DeVos and Chao are cowards. Why resign now? Stay and push for the 25th. If he tries to fire you, good. It's more of a statement than a toothless eleventh hour resignation. Absolute cowardice. The path of least resistance. All to be more accepted in their elite social circles. "Oh, that Trump was awful, that's why I had to resign." "Hmm. Yes. Quite."
Rats fleeing a sinking ship comes to mind, but in this case they could actually do something if they stayed. Awful, awful people, and I hope no one forgets it.
I think DeVos would not push for the 25th. Chao would certainly have thought about it. I take at least her resignation as a sign that she does not see a way forward for this and that there is no majority in the cabinet for it.