This is a question I have asked myself several times. I look at Ryanair and personally, I don't think that is sustainable. When you consider the way they have been operating over the last few years, i.e. offer cheap seats and make money from ancillary revenue, expansion and aircraft sales then I can see stumbling blocks.
Ryanair want to charge for everything and pay for nothing. What happens when there is nothing left to charge for? In 2004 you could get a return ticket to Spain for £25. This including the honour of checking in at the airport, paying by card and having a bag in the hold. Today, checking in, paying by card and having a bag in the hold alone will cost you £50...before the price of the flight even comes into it.
Airports (for example Manchester) are starting to get wise to MOL and have told him enough is enough. They wont prostitute themselves to him anymore. He's almost got nothing left to charge the passengers extra for. Fares will have to go up. The speculative traveller who only flies FR because it is so cheap will be lost.
Time will tell I'm sure.