Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health. In adults, a body mass index (BMI) over 25 is considered overweight, and over 30 is obese.
Doubt this will actually happen. If Southwest had larger more expensive seating options, not double the price it could work, but there will be constant arguments from the fatties. Two economy seats on say United or American are more than one 1st class seat. So they're going to argue the 2nd seat should be half off. That would destroy the airlines margins.
It has to be you're either not fat or you don't fly rule to be honest. There are trains, buses and cars still. Flying isn't a right. There are height and weight limits for all sorts of things. If you're boating and there's 8 people that weigh 350lbs you could easily be stopped by DNR or the coast guard and fined and have to return to shore if over the boats weight limit.
Fact is that in an emergency they're a huge (literally and figuratively) safety hazard if you need to exit the plane quickly. I'd have no problem asking an FA to be moved or have the fatty removed from the plane for mine and everyone's safety. It would cause a scene, but I don't care.
https://www.who.int/westernpacific/health-topics/obesity?source=patrick.net
BMI calculator:
https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm?source=patrick.net
https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html?source=patrick.net