The guaranteed rate of return is a crucial point. (There are admittedly also some tax reasons why not paying off the mortgage early can be relatively beneficial.) However, way too many people look at the overall stock market returns over the past few years and conclude that anyone who pays off their mortgage early is an idiot. By that logic, anyone who could should have taken out a HELOC and bet it on the market.

It would have indeed worked out over the past few years. The conclusion for guaranteed or very low risk investment returns is far less obvious and probably has more to do with liquidity and psychological factors.