Currently airline passengers don’t have to pay for their carbon emissions, but I doubt that’s gonna last for much longer (we are in an emergency after all). And I’ve seen elsewhere in this thread that the emissions are likely gonna be somewhere between 5x and 10x of normal sub-sonic flights. The price of this extra carbon emission will probably be something that even affluent passengers will want to skip.
Starting to tax airplane fuel would be an important step towards reducing the CO2 output. Possibly that would trigger a switch to synthetic and carbon neutral fuels.
Subsonic flight has become much more efficient, cheaper and when paying for higher class, more comfortable. This is one of the reasons often mentioned for the economic demise of the concorde. The audience willing to pay $6000 on a concorde ticket, could now spend $4000 on first class subsonic with a seat that fully reclines into a bed. They can fly a little longer at night but sleep the whole journey.
Wendover did a good economic analysis of the concorde: https://youtu.be/n1QEj09Pe6k
At least.
Even if you think price is a barrier, think about how many more millionaires and billionaires there are in America and Europe today than there were just 20 years ago.
Tack on our society's rediscovered fascination with conspicuous consumption ("influencers"), and I don't think filling seats will be the problem today that it once was.
