Nothing, you can't just run bus routes nilly willy and have to register changes weeks in advance and they don't have any driver's in the first place who I can't imagine will be happy to be bailing out their friends on the rail when morale isn't exactly 100% as it is.

Arriva's X66/X67/X3/X4/1/7/23/24/685 and Stagecoach's X24/1/20/36 are the routes if any which will be affected the most rather than GNE routes which don't really run anywhere near the Metro; the only route really is the 9.