If you believe market growth will outpace what it costs you in interest, it can be more efficient to use the money you would have paid towards principal and invest that in the stock market.
After 30 years you’d still have the full principal left on the mortgage, but that’s a long enough timespan that the market should average decent returns and you should be able to sell your investments you made with the difference and pay the entire mortgage off if you wanted to.
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