I took out an Endowment mortgage 25 years ago. interest rates were high and so I expected a big lump sum on maturity. However, as interest rates dropped then my endowment was less "secure": however, my mortgage payments to the building society also reduced significantly so I paid the money saved on the interest payments directly into the mortgage (my BS were happy with this).
The result was that although the endowment did not reach the planned level, the total owed on the mortgage had reduced considerably and I made a small "profit" when the endowment matured and the mortgage paid.
Moral of the story... Low interest rates worked both ways. By paying out the same each month (even after interest rates dropped) the mortgage remained secure.