A bunch of great points, but i think this is the biggest one. During the heyday of piston ga, airline travel was expensive. It was also more dangerous and slower than it is now.
Also bear in mind the cars of the 60s & 70s. Again, slower and more dangerous. Also less reliable and harder to drive. Air conditioning was an option. Cruise control was not common and barely worked when it existed. Now we have cars that drive themselves faster than a 150 can cruise.
The edge case between car and airline used to be way bigger. People understood carburetors and magnetos. They didn't have 65" tvs hooked to ps5s. There's a zillion reasons why piston ga is much smaller than it was then.
The price problem is part economy of scale, part trial lawyer, and part that physics hasn't changed. We can't build a piston airplane today that performs significantly better than one built 50 years ago. Since those old planes can be updated with the latest avionics, why buy a new one?...which loops us back around to the economy of scale part.
Labor has a lot to do with it, too. I didn't appreciate how labor intensive building an airplane is until I started doing it. You can build an rv10 for 150-200k, but it you paid $50 an hour for the labor involved, you'd double that. Add in liability, overhead, and profit, and that new Archer doesn't look so overpriced.