Ecoboost engines are the volume leaders in the F150.
The 7.3 engine was designed to fill the gap between the 6.2 and 6.7 diesel. It was designed to go into fleet and commercial vehicles, not consumer vehicles. Fleets and commercial operators generally tow/haul more frequently so unladen mpg wasn't a concern. Ford wanted an architecture that was more durable, provided reduced maintenance, and provided good mpg while under heavy loads. The Super Duty chassis vehicles aren't subject to CAFE because of their weight.
For the 6.8, it's probably a downsized 7.3 with some changes for consumer applications to boost mpg. Ford doesn't have a decent v8 alternative for the Ecoboost. The F150 moves enough volume that having a higher output v8 likely makes sense. It would probably sell more than the 3.0 diesel, and costs can be spread out by adding it to the Mustang. Both platforms likely have some consumers for whom mpg is secondary, either because they tow frequently or the car is not a daily driver. And both platforms will have more fuel efficient alternatives to the 6.8 available.
I'm not convinced that we'll see the 6.8 in the Expedition. The mpg hit would be too heavy, for virtually no performance improvement over the 3.5. The 3.5 HO already offers superior hp/torque to what the 7.3 offers. I average 15 mpg around town.I can't imagine what the 6.8 would be rated. I think a rating in the low teens would be a turnoff for many people.
Personally, I would rather see Powerboost offered. More power, better mpg, and more functionality for camping.
7.3 comes in all trims not just fleet and is only in the f250.
Ecoboost will eventually be replaced on full size trucks and go into crossovers exclusively.
The 6.8 will be offered in the f150 only.
Pushrod engines are back at Ford for a reason. Ecoboost is not the future on Ford trucks.