Your pay by weight pricing sounds fair, but in practise it would be pointless if fair. The vast majority of weight of a plane is not the passengers, and certainly not the variance of weight amongst the passengers.
A 777-300ER is 170,000kg empty. Divide that equally by 350 passengers and you get 485kg per passenger. We'll compare a 75 vs 100kg passenger which brings the totals to 560 and 585kg. ie a 33% passenger weight increase only makes a 4% total weight change. Note that these are best case numbers and using real world ones is a heavier weight to allocate, and even more trivial weight change per passenger.
You know what's even lighter than passengers? Their luggage. And yet you get to pay steep premiums for going over the usual 23-32Kg limit. If you're curious what overcharge we're talking about see here [0].
This puts in context what kind of deal a 150Kg person gets compared to a 50Kg person. They pay the same price, one gets to carry 100Kg of extra weight. And again, this is a treatment that is only extended to overweight people. A very tall person or an invalid get squat in terms of "good deals". It's only discrimination when we're talking about extra kilograms.
Weight on an airplane is always a major consideration. But even if it weren't, fairness doesn't always have to come attached to profit.
> steep premiums for going over the usual 23-32Kg limit
That means oversize or special handling due to size or weight.
Also most air freight goes in the belly of passenger planes - more than goes in dedicated freighter aircraft. Your luggage is competing with that space. In addition it costs more money to handle luggage vs not handling it all.
That simply means slightly heavier luggage that gets over-charged by the Kg once you go over a weight that's still a fraction of the passenger. So a 10% increase in the weight of a 23Kg bag gets charged $50. A 200% increase in the weight of a passenger is on the house.
Almost everything carried by planes - freight or fuel - gets charged by size and weight. The only thing that gets charged exclusively per seat is the passenger. For every passenger that weight 50Kg a plane can carry 100Kg of additional cargo compared to a 150Kg passenger. An increase in average passenger weight of 20Kg for a plane carrying 500 passengers means 10.000Kg less anything else it can carry or more fuel burnt. Both of these cost a lot of money.
Yet airlines are willing to carry the extra 100Kg for free probably because of the outcry coming from the millions of overweight people around the world screaming "discrimination!". Still they are not willing to give 1" of extra leg room for free or larger toilets although there's nothing one can do about it if they fall into these categories.
I think my point stands. The actual algorithm used to calculate is not important for the discussion as much as the concept: it's a fair way to charge for such a service that has hard limits for size and weight.
I'm not forgetting about fuel, and gave the most optimistic numbers I could. Weight for premium passengers (eg bigger more complex seats, more floor space, bigger toilets, more galley space, equipment and food/drink) should be allocated to them, not shared evenly. I also don't think it is fair to allocate items like duty free carts, or what belly cargo is using. At the end of the day if you do a more realistic and fair allocation (~half the plane is not used by economy passengers), especially what people have control over (I didn't ask for duty free, dead heading crew, alcohol etc) then the weight differences between economy passengers still are not significant compared to the big picture.
Your example is not entirely appropriate because first you're comparing 2 classes of paying customers: the ones that paid a lot, and the ones that paid a little. Then you compare a service that's provided for all passengers equally, like the duty free.
My point was being overweight is free, being "overheight" or an invalid will cost you every time. I have to pay a lot more to get the same level of (dis)comfort that everyone else gets for free.
It's the kind of discrimination that people just let slide but it's discrimination nonetheless.
i think we are talking past each other - my point is that passenger wight vs total weight is not a very relevant metric - you need to think about in the context of payload [https://commons.wikimedia.org/wiki/File:Takeoff_weight_diagr...] which can be as little as ~10% of MTOW
A 777-300ER is 170,000kg empty. Divide that equally by 350 passengers and you get 485kg per passenger. We'll compare a 75 vs 100kg passenger which brings the totals to 560 and 585kg. ie a 33% passenger weight increase only makes a 4% total weight change. Note that these are best case numbers and using real world ones is a heavier weight to allocate, and even more trivial weight change per passenger.