Japan Rail has an interesting approach to the reserved vs. open seating debate: Most Shinkansen trains are 100% reserved seating but some of the trains have several cars with non-reserved cars.

If you buy a non-reserved ticket it costs less and you can choose any train which has non-reserved cars (i.e., the next available train whenever you get to the originating station). Then you can choose any seat in any non-reserved car. If all the seats are full, you have to stand or wait for another train. It's similar to riding a city bus in that way.

Seating on the Shinkansen is better than any airline's premium economy, with more than 40 inches of pitch and more width than an Airbus narrowbody. Bags travel free, but you have to haul them to and from the train. It's pretty much the opposite of a domestic flight in Japan.

If Americans were as compliant to rules as the Japanese, Southwest might have had a realistic option to offer a mix of reserved seating rows and open seating rows.