I won't downvote you but I will disagree: airlines already seek to optimize revenue per passenger. And to that end their best benefits and service are targeted

I won't downvote you but I will disagree: airlines already seek to optimize revenue per passenger. And to that end their best benefits and service are targeted towards high profit business and first class customers. For those there will be no paying by the pound because the miniscule increase in profit cannot be reconciled against the risk of losing their business and will most likely be waived anyway as a status benefit. However for the no status / low status passengers although in my opinion there's a baseline revenue target per seat airlines would like to hit and therefore we probably won't see discounts for skinny customers or children, penalizing overweight passengers for an added revenue stream seems entirely realistic. Airlines already hit no status/low status customers with a long lost of contrived fees for everything from picking your seat, slightly better legroom, boarding position and luggage count. Piling on another fee would be profitable and no doubt heartily embraced if it can be disguised as a safety concern.