I get what you are saying and there is some truth to it...but there was also a lot of luck involved in your example (or your a genius at predicting the market)
We picked up a used 2008 V10 F250 4yrs ago for $10k. Looked up and similar trucks are currently going for $15-18k...truck prices are crazy high right now. In a more normal market, you used 14yr old truck might only be worth $8-12k (hard to say as you didn't provide mileage or condition info)
At the same time, Fall 2008 was in the middle of the financial meltdown and they were practically giving away new trucks. BIL picked up a binged out diesel F250 for $38k around that time. I think the MSRP was something like $65k.
The timing of your purchase & sale, could easily have saved you $30-35k.
Resale does play a factor but the folks saying you get 100% or more of the diesel upcharge back are being unrealistic. Particularly as you get into the 15-20yr old range, the body, suspension and little odds and ends are likely pushing most people to retire the truck...not the power plant.