I'd guess that manufacturers might prefer to keep churning out big NA motors, but like you said, the MPG looks good on paper for the small displacement turbos and they have to meet CAFE standards. At least these turbo motors put out some nice torque low in the rev range, which is pretty nice for typical driving.H6 would be a nice option. While the numbers look great on paper for all these force induction 4s the unfortunately reality (especially in vehicles this size )are its working extremely hard to keep up with what 6 and 8 cylinders hardly break a sweat with. Short term will be fine but the real question is what kind of repairs or replacement are you gonna have to deal with when you start approaching triple digits or even sooner. I feel like auto makers like throwing big power numbers and higher MPG math on paper because it sells and not because its actually better. They just hope your out of warranty before it shits the bed
I've always wondered if there was an H6 available in the Ascent if the front overhang would be even loooonger? It kind of looks like another row of cylinders could fit in the engine bay though. I haven't poked around the 3.6 Outback to see if that motor appears shoehorned in there.