Then prices would fall to the point where people could afford their homes, or governments would run massive surpluses and could lower their tax rates to remove the burden, or some combination of the two effects. It would remove the disincentive to trade homes which is bad for everyone: if I live next to your job and you live next to my job we'd both be better off if we bought and sold each other's houses, but at the moment we're discouraged from doing that because our tax bills would come due.