I know what you are saying, but there are some added layers of complexity to this where the marriage analogy falls short. Kirk is likely close to retiring soon. On some level, he knows Goetz and the donors need to actively plan for that day.
Plus, one parent already excommunicated the oldest son, and the spouse still chose to stick around.
So there is some understanding that this is a business arrangement, not a marriage. And it includes third party decision making (donors). If Kirk was younger with more years left to coach, maybe he'd tell Iowa to shove it and go take his talents to another university.