Don Campbell and your reader were both misled by the word profit. Profit is the selling price, less the original price paid, less selling expenses. Accelerated payments are really a form of indirect savings and not increased profits.
By accelerating their payments the young couple are giving up other opportunities for using that money. Let us assume one of these is putting the $100 each month in a savings account earning 5% per year. After five years, they will have $6,800 in this account. This is $943 less than accelerating payments against the mortgage.
A second choice would be to accelerate payments against debt that is costing them 12% year, say a car loan. On the same basis as the mortgage acceleration, after five years, the debt would be reduced by $8,167. This is better by $424 against the mortgage option.
Of the three options, the best opportunity is to accelerate the car loan payments.
Don Campbell replies:
When in his question my letter writer used the word "profit" when he clearly meant "equity," my first instinct was to change the copy accordingly. The second instinct was to let it stand, but insert a reminder that the two words are not synonymous. The moral here is: Go with your first instincts.