Of course it is sustainable in the long term.
People work on the assumption that LCC will still offer the same service in 5 years time which is a nonsense. Funny how people forget how Legacys were 5 years ago and the dimunition in quality that has occurred, its a two way street where the LCC's move up the quality ladder. Anybody who feels it is impossible then have a look at Skoda once you changed people at the Top or Tesco 25 years ago.
FR's Michael O'Leary accepts he is not the person to be running the airline when it stops its aggressive growth phase circa 2012 and needs someone else in charge.
FR and others have shown that there is no need for an airline to have thousands of staff to do everything when you buy in services that you need or seek to minimise their costs.
Coming out of recession I expect fares to rise BUT there will still be £1 fares off peak on certain routes at certain times.