In all actuality, the 3.5L Ecoboost gets pretty good fuel mileage compared to engine with similar performance and capabilities. In the last PickupTruck.com half ton towing shootout, the 6-speed 3.5L Ecoboost got 11.3 mpg towing 10,100 lbs while the 8 speed Ram 5.7L Hemi got 10.7 mpg, and the Chevy Silverado 5.3L 8-speed got 11.1 mpg. The only one that beat in towing mpg was the 8 speed GMC 6.2L at 11.6 mpg although the 6.2L does require premium fuel at all times. Slap an 8 or 10 speed in the Ecoboost like the other trucks had and I bet it would have out done all of them. The only truck to beat the 6 speed 3.5L Ecoboost in loaded performance in those tests was the 8 speed GMC 6.2L as well by about .3 seconds in the 0-60 and .3 seconds in the 1/4 mile.
These results were the same in the Canadian truck King challenge as well. So saying that it uses more fuel than a V8 with similar performance and capabilities is mostly false from all of the tests that I have seen. Turbo charging is no different that today's V8s with multi-displacement systems in that they both vary the effective displacement of the engine. The only difference is that with MDS you are decreasing the actual displacement you have when power is not needed, and with turbos you are increasing your actual displacement when it is needed.
I think there are many reasons why there has not been a Ecoboost 4.0L-5.0L V8 to replace the N/A 6.2L. For one, you need an iron block to to be able to dissipate the heat of a towing duty cycle and do it reliably day in and day out. The aluminium block of the 3.5L Ecoboost is fine for the duty cycle of a half ton, and is why the 3.5L Duratec aluminum block was easily(and cheaply) used since it was already engineered. Ford currently does not have a small iron block V8 so they would have to make one from scratch. Remember, Ford took a big gamble putting the V6 Ecoboost up against the bigger towing V8s at the time so they probably didn't want to throw more money into it if they could avoid it if their gamble didn't pay off. They also probably wanted to see how well the 3.5L Ecoboost did in the half ton before they committed to building a completely new V8 block.
I also have no doubts that a small turbo charged iron block V8 is probably in the works as we speak. Why? Because CAFE standards will soon be applied to 3/4 and 1 ton trucks. Unlike what some have stated above, 3/4 and 1 ton trucks aren't apart of CAFE since they are over 8,500 lb GVWR which is why there aren't any EPA mpg numbers on their window stickers. There has been no incentive to make them more fuel efficient since most would buy diesels in the past if they wanted fuel efficiency in an HD/SD truck. Why spend the money on making a new engine if what you have is good enough versus the competition in sales and you are not getting penalized for it's dismal mpg?
Now that these trucks will be apart of the CAFE numbers in the coming years, Ford (and the other manufacturers) now has an incentive to make them more fuel efficient. Afterall, that is the reason why Ford put a 3.5L Ecoboost in the F150 in the first place and why other makes incorporated multi-displacement systems in their half ton V8s. To meet CAFE numbers.
There are probably some other reasons, but I think these are the major ones. However, I would not be surprised to see a small turbocharged V8 in the SD a few years after the new 2017 comes out.
