It does sound like Dan was blindsided.
It's the only way it could have gone aside from him stepping down on his own. If he'd been given notice in advance that this was coming, he would have rallied the troops to raise hell with the ADept and stop it from happening. Once it's done, it has to be accepted. They might be angry but they can't unring the bell and they have to deal with it. Pretty smart, if you ask me.
I think at some point even the staunchest supporters had to have doubts in his ability to coach in the modern era (post portal/NIL). It becomes a very fine line between being right and doing what's best for the program. The hangers on wanted to be right as keeping him was clearly not what was best for the program by any recent metric.
Same thing as Doc. He was not a bad coach for us, but year after year we were able to determine his ceiling and knew that we weren't ever going to get past that. Both had that early success that made it hard to let go for a lot of people.