I'm thinking that the problems Monarch has had is actually very similar to the problems that airlines like BmiBaby, Bmi Mainline and CityJet have had or are having. You could argue that it's inappropriate to compare an insolvent winding up with an orderly wind down, a takeover and a change in business plan, but actually these were responses to the same problem. And in fact Monarch's trading situation wasn't actually that bad; while it did report a massive loss because of an accounting adjustment the underlying trading numbers weren't awful.
You could say it's about prices, but Monarch competed on that and the people still didn't come in sufficient numbers. You could say it's about destinations, but other airlines survived the same problems. You could say it's about product, but all sorts of airlines survive offering all sorts of products. Here you could say it was about ownership, but actually the one thing Greybull did do is put money in (they just didn't change anything so that it wouldn't keep on needing more).
The problem isn't any of the things anyone has mentioned, but I would argue in fact that it has become more difficult for an airline to differentiate and market themselves (and that Monarch didn't do particularly well at this). I think the problem all these airlines have had is that they were smaller versions of things that already existed, and therefore served a dubious purpose that couldn't be effectively marketed due to economies of scale?