Your theory is wrong.
The Catskills were partially destroyed by air travel becoming cheaper.
But the Catskills are so close to NYC. And yet still very depressed in general.
I would think real estate should be higher being so close to NYC.
Probably many other factors in play, but for me this was a major one. It was really hard to justify going to the Catskills for a week in the late sixties and seventies, when I could get to experience England and/or France and/or Austria and/or Italy and/or Spain for three weeks to a month for a bit more money.