My daughter could have written your post foxie, and as you know, she’s a doctor. She frequently talks about the cost to the health service of overweight people.
Whilst I wouldn’t want to see people humiliated by being weighed at the airport, I do think it wouldn’t be unreasonable for them to give an appropriate weight when booking, but it would also need to include height as well, because as someone upthread pointed out, a tall person weighing the same as a small person would lead to very different needs in the seat.
To be honest, I think that airline seats have been getting smaller and smaller in recent years and I vehemently disagree with this. Some are as little as 17 inches across and frankly that is too small for comfort. Not so bad on EasyJet going to Greece, but a different matter when flying Singapore Airlines to New Zealand.