Aviation historically, has not gotten cheaper overtime. If anything, it is more expensive. Only rich people had planes 100 years ago and only rich people own them now and the trend line doesn’t indicate it’ll be different in another hundred.
Cheapest flight I've ever taken was 9.99 from Berlin to London — not sure if pounds or euros, but does it matter when either way it is less than I used to spend on a week of school lunches nearly 20 years earlier?
The comparison was conceptual thing to similar thing. Only the rich own and operate private planes, that hasn't changed. The fact that other, more sophisticated planes exist doesn't undermine the point.
There are also diminishing returns. And past leaps in lifestyles aren't guaranteed to continue the same trends, and can reverse.
The earth has seen several major extinction events. Arguably humans are the cause of the latest one. Doubtful we can sustain billions of us without an ecological system to produce our bare necessities, certainly not enough for a full life.
No, they are selling off seats that aren't taken. They are making more money since those seats would be empty.
Cutting costs would not be selling the seats and saving on fuel.
My guess is if flights priced in externalities they could not be cheaper than alternatives like trains. (Unless they're leaning on whales like first class to subsidize cheaper seats.)