I agree, in most user cases, the gas savings from E-Torque won't be realized for at least 7+years. This is similar to installing Solar on your house, where the savings realized aren't until at least 7-10 years, on average.
With that in mind, did you use the current average gas price in your area and then calculate a modest/conservative increase year over year? For example, you could take your zip codes average gas, and increase that price by, I don't know, 3% each year, for 10 years, and you'd have a conservative estimate of the cost of gas per year for your vehicle. I tried to find a national average of gas prices year by year here (
https://www.statista.com/statistics/204740/retail-price-of-gasoline-in-the-united-states-since-1990/).
For me, living in Phoenix, the ETorque wouldn't even work for several months because it doesn't operate in 100+F weather (tested this in August myself). So for my situation with operating temps exceeding ETorques capability, I would probably never realize any gas savings for the life of the truck.