Welll.... there is no doubt that any prolonged decline in income will be noticed by the owners pretty quickly and their future planning will be based on this.
They can cut costs and try and find more "innovative" ways of raising income - but I'd expect airline charges (and thus fares) to rise at the major airports. The LCO's will continue to play off the smaller airports as long as they can so expect more changes in routings from there.
In the long term you'd expect a steady decline in the smaller airports - losses, more value as housing, business parks etc so I'd expect more developers to buy in (think Manston, Carlisle, Doncaster, Teeside, Prestwick, Coverntry as airport s likely to disapear). This will reduce the LCO's options eventually I guess.....................