Hmmmm.........just had a thought. Most airlines have the "bag sizer" at the gate to see if your carry-on will fit. Maybe they should make POS try to sit in a bulkhead seat (non-movable armrests) and if they can fit, they can take their original seat. If not, SOL.
Of course, this is all tongue-in-cheek, but seriously, common sense needs to prevail. If a person doesn't fit in their seat, they need to buy another seat. If another seat isn't available, they need to wait until there is a flight with the proper amount of seats. Fair is fair....