I remember maybe 20 years ago we paid £300 plus for a return ticket to London, unless you were staying a Saturday night. Even then it wasn't that cheap. Now I can get a return flight for £30-40.
Well yes if you're selective about what fares you pick and present then you can make any argument. You'll still spend £300 return to London if you need to travel urgently; ryanair.com wants £289 to fly to London this Friday and back on Saturday.
In the 1970s the Caledonian Moonjet overnight fare was £13 return for Belfast to London. BEA's overnight Vanguard seats were even cheaper, I believe.
Those weren't loss-leader or sale fares, they were printed in the timetables and available for every seat on every eligible flight. Where are the equivalent guaranteed bargain fares these days? Gone, 'revenue managed' out of existence in favour of a few cheap seats marketed heavily to those susceptible to advertising.