Earlier quoted context omitted.
Airlines are often brought up as becoming cheaper with less regulations. While it was mostly manufacturers building more efficient aircraft, people are generally willing to trade leg room for a cheaper fair. That’s exactly the kind of thing regulators and the free market may come to a different compromise. Worse, but cheaper is often quite appealing.
During the era of heavy airline regulation in the US, the Los Angeles to San Francisco route was significantly (about 4x IIRC) cheaper than other routes of similar length. This was because it was the only (popular) route of this length that didn't cross state lines, and so the onerous federal airline regulations did not apply. So no, it wasn't just manufacturers building more efficient aircraft.
Adjusted for inflation and including fees domestic round-trip airfare in 1979 averaged $617.47 vs $366.92 in 2016 a 40% drop. Which looks pretty good, though there was that 1979 oil crisis which needs to be accounted for which complicates the inflation comparison.
Meanwhile Jets become 70% more fuel efficient between 1967 and 2007. As airliners use a mix of newer and older aircraft there’s a delay before improvements affect prices. But most of that improvement was front loaded so by 2018 domestic airlines where getting 58 pay passenger miles per gallon of fuel an unheard of number in 1979 which again also happened to be the middle of an oil crisis.