The concert analogy is close but not perfect because the problem is that grouping necessarily puts you into premium seats.

If they can sell aisle and window seats at a premium, you can't allow people to group for free without putting people in relatively more premium seats for free.

The scale of airline seats (on a single flight) and concert tickets is incredibly different anyway, and different forms of price discrimination are required.

Southwest is an example of general admission applied to airline seating.

By the time you're calling non-middle seats "premium", we've already lost. We should aim to have markets that converge towards abundance rather than maximum extraction through things like price discrimination.

There's no reason to identify with these scumbags. It's indefensible that they sell seats which then aren't actually big enough to fit a person, whether it be their legs or their arms.

There's no airline seat that can't "fit a person", that's hyperbole. Fit any person, maybe. Fit without touching the person next to you, maybe. Fit without negotiating who uses the armrest or shared spaces with a stranger, maybe. I fly frequently, I'm 6'2", I'm medically obese. I fit into a middle seat. Don't love it, it's not the nicest, but it's less cramped than any public transit seat and most theater seats.

By your definition, it would be fine for them to sell the seat that was your lap to someone else, since your body still has enough volume to exist.

I've definitely flown in seats where I had to spread my legs (eg into the aisle) so me knees were not pressing up against the seat in front of me. And no matter which seat I sit in, my arms generally hang over the armrests. Talking about "the" armrest nicely demonstrates how much we've been saturated with this nonsense.

As I said, I'd have no problem if they charged as a linear combination of your weight and the sizes of your seat. But rather the setup is to gouge an outsized price for a comparatively small amount of room, because they'll catch some people at the margins over a barrel.

The idea that everything that can be differentially priced should is really annoying and leads to BS like this where everyone's life is worse to marginally increase profits.

No, it doesn't, it's still a matter of luck. When you get to a general admission event, you might find that there's a seat in the middle two rows up, two seats together and then another seat three rows away. I've certainly experienced this; and I've experienced cases where I'm lucky and we can sit together. But it's very typical for parties to leave single seats between and for you to be stuck with scattered single seats.

General admission is usually either standing room or lawn seats. I can't think of a show I've been to that had GA tickets and capacity defined by the number of hard seats, for exactly that reason. I'd believe it could happen, just not commonly.

There are many reasons you might want to split up your group at a GA event, like say some people wanting to push up to the front. Or in your seat example, if your group values being closer rather than being together in further back seats. But that's your own choice and not a result of the venue forcing you.