I think they should do that, or something similar to what courier services use: a combination of weight and volume. It's common for kids under a certain age to already be charged less.

I believe that's illegal but might be mistaken. From an ethical perspective is also nothing the passenger can easily control

The delta between the typical obese person and my weight is much more than the typical weight of my carry-on bag. There is zero reason to charge for the carry-on bag if you don't also charge the obese person.

Precisely. What is up with the obese requiring 2 seats? Do they pay 2x the ticket or not? I need up to date information.