ROI or opportunity cost. I can continue to pay 3-4% on a few hundred thousand for a few decades. Meanwhile I have a few hundred thousand “cash in hand” that I can allocate to other assets/investments. I’m betting that my other investments will return more than my 3-4% cost. And since we’re talking decades I can choose when to unwind this allocation. As I near retirement then yes, I expect to pay off the mortgage just like I’ll move out of broad equities and in to bonds and dividend focused equities.