principal is whatever interest rate your mortgage is. It is not great - 2.5%-4% but that is your return. I don't know what you are saying when you say the principal doesn't get interest. You save whatever interest rate your mortgage is on the amount you pay off every year. I do agree if inflation is greater than your mortgage rate that is a negative return in real terms. If the rates were the same I would get Interest only on all of our loans, but rates are higher and interest only is not readily available on investment properties.