If you assume that you'll never have trouble getting enough income to make the monthly payments, and that investments will, over the long term, consistently return significantly greater than your interest rate, and that you'll invest the difference in payment rather than spend it, then sure, it makes mathematical sense to never pay off your mortgage. Just cash-out refinance it every few years. However, a job loss, or a prolonged market downturn, or a lack of self-discipline, or simply the mental benefit of being out of debt can tip the scales in favor of paying off your mortgage.