> The first class passengers are effectively paying 3x as much for the same outcome

Not the same outcome. They show up at their destinations fresh from a good night's sleep, having showered at the lounge. Their back doesn't hurt from trying to sleep upright in a tiny seat or schlepping a heavy rucksack.

If you have enough money you are ok with paying to get those outcomes.

Amtrak sucks in so many ways, but that said for almost a decade prior to COVID when I had the opportunity (roughly yearly) I'd travel Amtrak first class between Tacoma WA and Oakland CA (Starlight). Kind of a day's vacation going each way (and they'd sell you a tax free bottle of good wine you could wander around first class with).

It was weird, I would arrive in Oak / SF rested, grounded, not nearly as stressed... and without a flying horror story to share (although the "alien abduction" made a good story). [0]

At the end of the journey Amtrak sucked, but flying sucks more; so much of the suck is fixable in both cases but it never seems to happen.

[0] I'd rate your chances as about 1:3 something unexpected will occur. An 11PM electrical fire / "alien abduction". They put the train together wrong / in a hurry and first class was right behind the engines (they sealed the door with duct tape to keep the rain / snow / wind out), and at night going through the Cascades in OR there were a half dozen of us crowding around with our wine bottles looking out over the engines; very cool. A freight broke apart in the Cascades stretch and we sat in the bar car (drinking wine) and watched the maintenance crews humping connector knuckles from behind us up to the freight in front of us (some of those freights are over a mile long; did I mention we were basically on the side of a cliff?).

For very long international flights I could see it, but I also see people in the first class seats on shorter flights where it’s hardly worth it. (I assume at least some of them are upgrades.)

Also, checking a bag is not expensive.

It doesn’t matter how luxurious first class is I am never going to get a good nights sleep on a plane.

Most of first class is not paying their own money for those flights.