Hmm. So you don't disagree that cheap semirural airports with a population base and short flights are important parts of the model, but you don't think they are

Hmm. So you don't disagree that cheap semirural airports with a population base and short flights are important parts of the model, but you don't think they are related to population density? I don't see how those could be unrelated. Perhaps a simpler way to convince you of the importance of population density to today's LCC model: Take a look at a map of where Ryanair flies. Under the 58th parallel, on the entire European continent, there is only one place over 250km from a Ryanair airport! Look at all the tiny airports, Limoges or Valladolid or Brive or Osijek or anywhere really. Ryanair flies to 250 airports and they do it with hundreds of 737s operating like clockwork, one short flight to another, not a tiny regional plane to be seen. Meanwhile, Delta flies to a similar number of airports in the US (in the 200s) but the majority of those are with small regional jets which are way less cost-effective than 737s; if you're an LCC, that means that you can forget about flying there. Obviously Delta is flying all those regional jets not for fun, but because they can't fill a 737, because the local population is not large enough to support one. I don't see how population density is not a core part of the issue here.