Average ticket prices didn’t suddenly fall anything like 75% within 2 or even 10 years of reduced regulations. Which should make it clear it wasn’t onerous federal regulations causing that difference.

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.