In all of your complexity, you seem to fail to account for the "control case" of GP's parents renting the whole time. You can't reasonably "charge" the $970K for the next 30 years of rent against their "profit" from their ownership experience.
What you've essentially proven is that, "In order to live in their original house for 25 years plus an additional 30 years of renting, they will have out-of-pocket housing expenses." That finding is air-tight, but also unenlightening.
What would it have cost them to rent for all those 55 years?
What you've essentially proven is that, "In order to live in their original house for 25 years plus an additional 30 years of renting, they will have out-of-pocket housing expenses." That finding is air-tight, but also unenlightening.
What would it have cost them to rent for all those 55 years?