Every major airline also sells their tickets based on transporting one human passenger, end of story. I'd be open to a price based on some linear combination of pitch/weight/etc. But shrinking seats so they don't fit actual people, and then upcharging for a few inches of "premium" space is downright fraudulent. It might only be 20% over their fantasy "full fare" prices, but it's a larger proportion of the actual prices most people pay.

Regular economy airplane tickets are a lot cheaper than they used to be N years ago for any N.

I fit well in a regular airline seat. Why should I pay more because a fatter or taller person wants to sit more comfortably in an airplane without paying extra for a seat in economy plus designed for that exact purpose?

"Why should I pay more" isn't really an argument in the context of a company that is trying to extract the maximum price from every single person. Someone else's price going down will not make yours go up.

But as I said, if the variation in humans is really that large, then charging a linear combination for depth/weight based on actual costs would be completely understandable.

For the current state, let's take a quick look at say United [0]. A regular ticket from BOS to LAX is $139, which is in line with the general range one would expect to pay [1]. The upgrade to "Economy Plus" is $99. So for a pitch increase of 20%, the price is 70% more?! This is price discrimination based on immutable physical qualities - straight up gouging.

[0] I chose what I thought would be the worst offender here

[1] and has been roughly the price for well over a decade, disregarding the new bag fees

And they reserve the right to give that seat to another customer at boarding time, so despite having paid extra, you end up with a seat you literally can't sit in for the flight, leaving the flight crew with a problem the ground crew have created. On top of that, the only refund you can get when they do this is the 10% surcharge for the upgrade, not the price of the seat you now can't use.

I fly 75,000 per year on United and never once was my economy plus seat surrendered at the boarding gate involuntarily. Getting gate-bumped is extremely rare based on passenger miles flown.

And yet it's happened to me twice, and I specifically avoid long-haul flights because of my height. This probably depends on the airline.

It was more like 50~100% extra last time I checked. Perhaps because I always fly international, and the economy plus seats are more popular on those?

    Every major airline gives the option of paying 10%-20% more and getting THE SEAT PITCH THAT USED TO BE STANDARD A DECADE AGO.

Fixed that typo for you.