Got it - you won't draw any line; that way nobody in particular is responsible or has to do anything. This sort of maneuver is surely just what the author means when he says a central tactic of the 9.9% is: "the pretense of being part of the middle as one of our strategies for remaining on top."

The OP argument in a nutshell: "In the United States, it’s the children of the bottom decile and, above all, the top decile—the 9.9 percent—who settle down nearest to their starting point."

So how would using just a function, so as not to embarrass any one class, fix that?