Does the global economy really need passenger planes? Tourism economies, sure, but I would guess most of the economy is dependent on trains, trucks and ships?

Yes. Take the SF Bay Area. It would be a lot harder to convince smart engineers from around the world to move here if they had to take a ship whenever they wanted to go visit their parents and relatives back home.

People certainly immigrated before the airplane, but in general those were individuals without many options at home. Skilled immigration really took off during the Jet Age.

Yes, meetings still happen face to face for business to get done. Giving up air travel, is not a reasonable option for the world at large.

Giving up things like modern medicine would not be reasonable. Giving up air travel, or at least reducing it vastly, would be merely an inconvenience. Well yeah, giving up air travel entirely would be somehow a mess, but even there it is completely sure we can make it work.

In France the train is ~50 x more carbon efficient. We will not be able to keep emitting like crazy forever (we actually should have stopped completely already :/ ) , especially when reasonable alternatives exist (the train if going not too far, teleconferences, ...). If we believe in the markets, the price of an air ticket should be maybe 50x higher than what it is today, in order to stop the insane emissions. Alternatively there could be an authority who decide if we have a good reason to take the plane.

If trains are so much better, why is it cheaper to fly?

I'm not sure this is cheaper to fly, but part of the reason this is too cheap is that fuel for airplane is not taxed...