A Diamond DA40 does around 28 miles per gallon (< 9 litres per 100 km) at 60% power cruise (5.1 gallons per hour at 124 knots). Sure, more than a car, particularly if you add takeoff and landing (though also note that in a plane you can go in a straight line rather than following roads). But it's not outrageously more so, and of course in aggregate much less than what cars consume now.
The environmental impact is not the main issue here.
> you can go in a straight line rather than following roads
Yep. You also need another type of transportation to bring you to the airport and back. Given the price tag I doubt the target customer would take the bus.
> But it's not outrageously more so
More than too much is way too much.
> in aggregate much less than what cars consume now
The aggregate value is totally beside the point since one could always make up some subcategory for which the aggregate is small. What counts is the unit value per person per year. Unless of course we assume it is ok for some people to mess with climate while the others do more than their part to compensate.
> The environmental impact is not the main issue here
The environmental impact is not the main issue here.