....while espousing excellence, it thrives on mediocrity....
If the truth be told, I think most of us work at institutions where this is the case.
Although the specifics at my place differ, I think a common theme is the powers that be are not ready to walk the talk in providing leadership, support, and resources to move from mediocre to excellent. But I suppose talking about excellence is easier than actually doing something to get an institution there.
OMG, Zharkov is so correct here. So painful, but so true!! This from him belongs in the HOF.
OP, I'd say you can resign from the chair-ship when this semester ends. You've done enough, and you'll give everyone enough lead time to figure out a decent-enough new chair. If you're miserable, definitely cut your losses.
But there's no possibility that the dean will pull a fast one and dump the chair position and put your dept. under the control of a director, is there?
Quite a few institutions are trying to do that sort of thing so that deans and prezzies have more control.