During the last U.S. recession Southwest was the only carrier to post a profit whilst the big boys were loosing millions. Here in Europe I reckon Ryan could survive together with perhaps one of the other three. My bet would be on Easy (only because of the financial clout behind that company) with Go and Buzz dropping out.
When there's a recession people might be watching the pennies but travel doesn't stop. A good portion of the business community may opt for Y class on the majors or even switch to the cheaper "no frills" (and more importantly, "no restrictions") carriers.