The only engine any of the major manufactures offer in their HD trucks is either a big V8 or a diesel. Ford has never offered an Ecoboost in their HD line. Must be a good reason they still haven't five years after it was introduced in the F150. Not buying the fuel economy argument either. As Ford has proven it's either eco or boost, you don't get both. Since many HD owners frequently tow, it's not going to be eco.
NA engines are generally cheap to produce and reliable. It only cost FCA about $1500 to produce a 5.7 Hemi so good bet the 6.4 cost is still below 2k. Since HD's aren't held to the same fuel economy standards there little good reason to stir the pot. Small(er) FI engines generate a lot of heat and many run some pretty crazy cylinder pressures. To make good horsepower reliably it's going to cost manufactures a lot more money in the end. As the saying goes: Fast, cheap, reliable, you can only pick two.
FCA has stated the 5.7 is going to be discontinued in the HD trucks, which makes a lot of sense. But they also stated the Hemi end-date had been pushed back to at least 2021. Sources also stated there would be some upgrades to the 6.4 in the near future. If FCA did nothing else but offer the 8-speed trans on HD's most of the common complaints would go away. If they bumped HP and torque considerably, plus a new trans, who would really need turbos?