One of the points of the article is that the vaporfly isn't the first shoe to do this, in fact, it has been going on for decades now. Moving the tech back to 2

One of the points of the article is that the vaporfly isn't the first shoe to do this, in fact, it has been going on for decades now. Moving the tech back to 2015 would not achieve your goal of no energy return. So if there is some energy return allowed, how and where can we draw a line?