It also means that you're often still out actual money if you use award miles.
It also means that you're often still out actual money if you use award miles.
I'm not even talking about pay-by-weight as was famously tried between pacific islands. Nobody wants to have someone spilling over the armrest into their seat, and I'm sure plenty of people who are wider than the seat would like to fit without going first class. I'm not even so unusually sized, but cannot sit in the aisle without being hit by every person and trolley passing by.
And compliance is hard for passengers, because you have to call in to book the special case, and who wants to call in?
But theoretically, a passenger that will encroach on an adjacent seat can pay for the extra seat (I don't know if they need to also pay for seat assignment to get two seats next to each other), and then if the flight doesn't actually sell out, the extra seat fee is refundable. But when you actually board, people will see the 'empty' seat and try to sit in it, even though you paid for it. Etc.