I'll imply those things. If you don't fit in the seat, you should have to buy two seats is a not very controversial opinion on the internet IMO. I think that opinion basically violates all of your "surely"s already.

Where do you draw the line? A 250lb person probably mostly fits in their seat still, but at some point a person is just physically going to take up two seats. Do you really think the airline should be responsible for flying them in business class (premium economy doesn't give you more width on most/all airlines)? Does it matter if their weight is due to a medical condition or just laziness? What if they're so big that even a first class seat won't contain them?