I think it's more like saying it's not worth the cost to scrap all existing airplanes for a new airplane design with a 10% gain in fuel efficiency, if a modific

I think it's more like saying it's not worth the cost to scrap all existing airplanes for a new airplane design with a 10% gain in fuel efficiency, if a modification to existing ones will make them slightly uglier, heavier and still give you an 8% gain. Some "purists" would much rather see all-new planes flying around. They would point to any number of deficiencies in the old airplanes to make their point and they would try to minimize the extra cost of replacing all the old airplanes (you're going to have to replace them in 10 years anyway, so why not start now?).