You have paid a price to go from point A to point B. The airline industry has determined that to that that, they need to seat you "safely." That's why there are seats in the plane.
You may have agreed to pay $200 to reach your destination, but the person next to you may be paying $350, because s/he didn't buy a ticket until this morning. Does that mean s/he should be able to lean over into your "space," because you didn't pay as much for your ticket?
Of course not. You agreed to pay $200 to get where you want to go, s/he agreed to pay $350. You both got what you wanted. So what's the problem?
If the other person hangs over into your space, ask him/her to move over. If s/he can't, then ask the FA to move one of you.
If that isn't possible, try suing the airline. First, try to figure out what your claim is. Answer: You don't have a legal claim. So live with it.
The truth is that the airlines have reduced the size and spacing of their seats over the years to squeeze more passengers on board. At the same time, the population is getting larger in size. An enterprising airline will use these facts to their advantage, if they can figure the financials and the marketing cleverly enough.
It's not really my premise, it's an assumption I made about the airline's possible argument. As I said, you either treat people as people, equals, or as cargo, by weight or space taken up. I'm not a lawyer, so I don't know what the fine print says. But I'm just saying that an airline might attempt to use the 'space per passenger' rule as justification for their new policy.
One solution is to have a set of seats that are wider, but are ONLY to be used by those who NEED them. Otherwise, if some skinny guy gets to sit there he won't be resented by the rest of the cattle in coach. Airlines have the number-crunchers, they can figure out a min/max number of such seats on the plane.
It is not a cut-and-dry issue, I agree. If an overweight person doesn't infringe on my space, nor get any extra for free (that he doesn't NEED), I'm fine. But if you give someone who takes up SLIGHTLY more than one seat, a free seat, you are also giving him the FULL use of that seat for free, which is just plain unfair. It means he can put his possessions on it, stow things under it, etc. A solution to this would be to allow a middle seat as 'fair use' by the passenger on either side of that seat. The overweight person would use it for his extra girth, but the 'normal' weight person could put headphones, a pillow, a book, or whatever on the same seat. For free. Of course, this is a luck-of-the-draw thing, so other passengers won't all have an empty seat next to them, but that's just one of the facts of life.
Man, I never thought I had so much to say on a topic I've barely even thought of before!
But the comfort issue, if not addressed by the industry will surely be addressed some day by dot-gov. Whether we like it or not.