Just to throw another bit of information into the pot. I recently flew to Europe on a British Airways 787 with my son and a couple of his friends. As the day to travel drew nearer, I started to get nervous, as the 787 is just about the worst plane in the air in terms of seat width. So I called British Airways and asked about buying two extra seats, which they supposedly sell at the same rate as the original purchase, so that we could sit in two rows of three with empty center seats. The guy on the other end of the phone acted like he had absolutely no idea what I was talking about and after a while told me I would need to talk to another department. I just said "never mind."
I ended up booking the very back row on the port side, which only had two seats instead of three, plus two aisle seats across and in front (for the record, these "twin" seats recline as much as any others). This worked out well enough. The back row had about 14 inches of unused space between the seat and the window, which was a nice place to put stuff. But obviously the airline could have installed some 23-inch wide seats in these rows and sold them at a big premium. It would solve the problem with no loss of passengers. Sure, extra expense in configuring a different seat, but since this configuration exists on every single 787 it shouldn't be that big of an expense and should more than pay for itself through premium seat charges.
At any rate, my point is that the airlines for the most part are just doing very little to deal with this problem. Most don't make it easy on the front end to buy extra seats. When they could easily offer a wider seat, they instead offer a super-narrow seat with empty, unutilized space on the side. I weigh 195 pounds, am 5-10, and have a 34" waist, so I am somewhat "fat" but not super-fat. I find a 17-inch seat width absolutely intolerable on a long-haul flight. There is no way a severely overweight person can fit in and remain within their allotted space for six to 10 hours with only 17 inches of seat width.