This is a great article because it speaks to two often neglected points in the "should you pay off your mortgage early" debate.
The first point is that the purpose of money isn't to just maximize returns! In fact, I'd say that maximizing returns is a "sub"-purpose, at best. Really, we use money to make our lives "better" (in as much as money can make lives better--people over things, ladies and gents), by: purchasing gym memberships, buying subscription services, or paying off your mortgage early.
Really, we could sacrifice any one of these to have higher returns in the stock market, but we don't. Why? Because, again, we care about other things than just maximizing returns.
The second point is that debt is risky. People are so used to being in debt that they forget this point. When you take out a loan, you're betting that your future income will be enough to repay this loan. Depending on the size of the loan, as well as the length of the loan, that's a huge bet to make!
Mortgages are one of the largest loans we'll ever take out. It also takes a long time to repay.
During that time:
What's the probability that you will get sick and be unable to work for a period of time?
What's the probability that you will lose your job?
What's the probability that your partner (if you have a partner) will lose their job?
You have to take these risks into consideration when you're calculating the returns in the stock market vs. interest payments on the mortgage.
This is not an easy calculation to make, but it's helpful to remember one point: there ain't no such thing as a free lunch!
This is all to say that paying off the mortgage early is a great idea!