I think, at least in the US, that the flying demographic is getting older and wealthier in general. I have not researched this, but that have been my anecdotal experience.
I have multiple couples as friends that are over 60, comfortable financially, and ALWAYS fly 1st class, no matter how short the flight. That is simply what they do.
I was looking at the cost of upgrading a flight for a cruise a couple years ago and was informed by Viking Air Plus that the upgrade fee for RT flight to Norway in 1st class would exceed the cost of the 15 day cruise!!!!! I then asked, how many Viking customers do that? She said, "over 80%"
I was blown away.
The only flight I would pay the premium for would be when and if we go to Australia. That is a marathon flight, and I would rather just splurge for the 1st class for that length of flight.