Legacy won't "fail" as such....as wobble points out there are far too many people out there who will just refuse to ever use loco's, regardless.
What I think may happen, however, is a serious re-think of the underlying business model and strategy....BA for example using the premium end of the market as much of their cash cow are suffering badly, also with the generally poor service offered in comparison to alternatives. It will survive easily, but I doubt it will be recognisable as the same beast as it is now.