This seems overly pessimistic to me. While doubling efficiency may be impossible 10% is far too low as an estimate without a fairly short timeframe.
Weight reduction - Reduce the weight and you can reduce the air you need to move.
There are also conceptual designs for integrated wing aircraft that if they were easier to make and could be made to work with current airports might have further real gains.
These added to the engines and other factors mentioned probably make at least a 40-50% improvement possible (although not necessarily easy or quick to achieve).
This doesn't break the concepts explained or mean that there is unlimited room for improvement but 10% is not going to be the limit by a long way.
Weight reduction - Reduce the weight and you can reduce the air you need to move.
There are also conceptual designs for integrated wing aircraft that if they were easier to make and could be made to work with current airports might have further real gains.
These added to the engines and other factors mentioned probably make at least a 40-50% improvement possible (although not necessarily easy or quick to achieve).
This doesn't break the concepts explained or mean that there is unlimited room for improvement but 10% is not going to be the limit by a long way.