"Wonder what kind of MPG that 534cu/in beast got fully loaded?!
Of course, gas wasn't $3+/gal back then either..."
My father ran a service station in 1962 and he priced his gas around 31 cents per gallon. New cars ranged from $2,000 to $4,000 in that era so wages have gone up by about a factor of 10 as well so the real cost of gasoline has not really changed.
Back to the op's question, I think that it is just a matter of Ford not being able to make enough of the Ecoboost engines so they put the ones they have where the EPA hounds them the most on efficiency. I would do the same thing. As for durability, I have not heard anything that would discourage me from having one in an F250. It would be the pants off of my 460 in power and torque (230hp and 390lb-ft). I would be much more likely to by a 250 if an Eco-boost option was available. Right now I am waiting to see what the 2015 F150's have under the skin and I will probably go with one of them.