The central argument of the article seems to be that an air plane must spend energy for drag an lift, and there's a limit to how good you can get at those.

That's all fine in the current model, but other modes of transport are thinkable. For example an airship that doesn't need to spend energy on the lift, or a ballistic transport model (think ICBM) that uses flight parabolas around the curvature of the earth.

Ok, you probably wouldn't call their "airplanes" anymore, at least not by traditional terminology.

Update: oh, and you can always try to improve the constant factors involved, like the total aerodynamic drag and weight