Airlines publish seat width of their aircraft, one could argue this is advertising.
Surely this forms part of the sales contract? i.e. a provision of a seat with 18 inches width, with the implicit understanding that all of the 18 inches would be for the sole use of the passenger.
The lack of resolution by airlines of this not unusual situation, implies that it is a passenger issue.
So why not apply civil law?
At what point does the continual refusal of a person making unwanted physical contact, deemed an assault?
If the obese person does not leave the plane then sue them, at the very least for the % of seat for which you have been deprived.
Obese folk are well aware of the inconvenience they cause on aircraft yet they selfishly hope they can get away with it.
For those obese folk suffering financial hardship, perhaps the booking system can give them an option for a change of flight (that has two seats available) without charge.
Mickjoebill