What does 3 have to do with paying off your mortgage? First, funding your retirement, likely due to a match or to save on current high tax rates or taking advantage of a Roth, is done first. The payoff money would go in a brokerage account. Second, you’d have more free cash if you had the money that you used to pay down the mortgage. That’s the point of getting a better return on your money than the mortgage interest and basically having a nice nut of extra cash left over after paying off the mortgage via payments.
All that said, that was for the last decade. Paying off the mortgage in 2011 would have been a good feeling but over the last decade you had 14% annual returns or a 280% total return. For a 30 year mortgage, you could probably use the 4% to pay the monthly payment and still have gotten 10%+ (compounding) a year thus having around double your money at the end of the decade while making every payment. You might have been able to take half right now and pay the rest off, this paying off your mortgage with the decade of gains and still have the entire principal right now.
For this decade, I’d be more willing to pay off the mortgage because we’ve had such great returns the past decade, but last decade you would have killed it by investing the payoff and skimming the return to pay the mortgage.