Or some other drastic change in society.  Maybe people will become widely enlightened to this problem somehow.  We've mostly learned to accept germ theory; maybe we'll mostly learn to accept CICO, eventually (possibly in part via FPH's favorite mechanism, i.e. widespread shaming).  Or maybe a worldwide dictatorship will arise and (among other things) outlaw obesity.  Or maybe genetic engineering or gut microbe science or nanotechnology etc. will produce some physical method for directly protecting people against obesity, maybe by preventing digestion of excess calories from completing (so you could still eat extra food, but you wouldn't get nutrition from it) or by producing satiation or even nausea when appropriate to prevent a potential fatty from even wanting extra food.