Which leads to the interesting question of how do you refinance a mortgage. You have to pay the full “interest” no matter how long you actually have the loan, so I guess refinancing never happens ? You buy a $150K home on a 30 year mortgage, but owe $400K even if your life changes and you want to sell 6 months later ? That must work out great.
Mathematically, it would not be difficult to perform the same sort of calculation to a refinance.