This doesn't make a whole lot of sense to me. Maybe I totally missed the point somewhere, but as far as I can tell, it's basically saying that every airplane has an optimal speed for best efficiency, and going slower makes it worse, thus you can't make planes more efficient.
This totally ignores the possibility of using different airplane designs altogether. Build something whose optimal speed is slower, and you can make the whole thing more efficient.
Probably the best single-number measure of aerodynamic efficiency is the lift-to-drag ratio. For most airliners, 787 most likely included, this ratio is in the low 20s. The best machines out there are in the 70s. Of course, they fly slow, which is a tradeoff airliners don't want to make, but it shows that there's plenty of room for improvement.
Can we build a more efficient airplane? Of course! But we may not want to.
This totally ignores the possibility of using different airplane designs altogether. Build something whose optimal speed is slower, and you can make the whole thing more efficient.
Probably the best single-number measure of aerodynamic efficiency is the lift-to-drag ratio. For most airliners, 787 most likely included, this ratio is in the low 20s. The best machines out there are in the 70s. Of course, they fly slow, which is a tradeoff airliners don't want to make, but it shows that there's plenty of room for improvement.
Can we build a more efficient airplane? Of course! But we may not want to.