So, if I understand what your are saying, in your world the fair thing would be for the person who's seat is encroached upon by overflow from a huge passenger should pay to "reclaim" the encroached upon space? That makes no sense.
You in fact did misunderstand me. I never said it was fair of the airlines for the skinny person to pay to reclaim the space. I said in so many words, which I will now rephrase so that it makes sense specifically to you: the skinny cramped person would, given that he faces a five hour flight of unwarranted discomfort would like to turn to his neighbor and say, "listen, I'll give you ten bucks for you to remove that part of yourself that is encroaching into my seating space. Because frankly, I think the ten bucks is minimal cost compared to the amount of added comfort I'll gain." Under his breath the skinny person is saying, heck I'd pay double that to reclaim the comfort.
The point is, once the person is in that situation of being cramped, that is one alternative that pleases them more than suffering through five hours of flight from hell. Next time they fly they'll take the issue to management before getting on the plane.
The person who's taking up more than one seat should be the one to pay for the 2nd seat, not the displaced passenger.
You're right. Hindsight is twenty-twenty. But it does the skinny person no good when they find themselves squished. Think of it this way. If an obese person sitting next to you offered to pay you for the space that he wanted to encroach upon, would you take the ten bucks or say, "no way". I think most people would say no.