there are so many complications with that policy. It slows down boarding while the gate agent determines each customer's fare. Gate agents would have to take time to explain why the customer isn't allowed to carryon and everyone else is. (Gate agents already have to waste time explaining how the customer agreed to no voluntary changes or seat assignments as it is currently.) And what if the customer has an assistive device in the carryon? Lots of complications when it's not that much extra cost.