I do enjoy the charts and banter. For those that are interested The Mortgage Professor has a gob of threads on this here:
https://www.mtgprofessor.com/ArticleCategories/Prepayment_of_Mortgage_Balance.htmlThis thread seems particularly relevant:
https://www.mtgprofessor.com/A%20-%20Early%20Payoff/another_look_at_mortgage_repayment_as_investment.htmIt is entertaining to see all the other variables and assumptions tuned, but to me it pretty well boils down to:
1. If you can get more investing in other projects and your time horizon is longish the better financial decision is to invest in those projects assuming they have manageable amounts of risk
2. If you can't get more investing in other projects you should pay down your mortgage. With rates as low as they are right now this seems like it is very easy to do for most people with moderate timelines
3. If your timeline is short for whatever reason you're unlikely to be able to stomach the extra risk and principal repayment or simply hoarding the cash may be better options depending on your situation
These threads are impossible to follow because this is a debate that is impossible to have in the abstract. What is the owner's time horizon? What is their tax situation? How stable is their job? How is the rest of their portfolio allocated? How marketable are their skills on the open market in all market cycles? Are they the only breadwinner in the family or does their spouse work? Do they come from a wealthy family? In short, what is their tolerance for risk?
Completely rational beings can have very different answers to these questions. Finance is SOCIAL science. Saying that someone should "do the math" necessarily has to factor in a risk discussion. There is generally data to describe observations of index funds or other homogeneous investment alternatives. However, there isn't generally data in threads like this to describe the investor's risk tolerance. This risk would necessarily involve quantifying many if not all of the items listed above and probably many more.
It's fine to have useful guidelines here, but absent more precise data about one's personal situation I am not sure how anyone can prove anything.