It also means an end to the barmy and crazy idea of concentrating the all the expresses on the WCML, MML and ECML on to one single pair of tracks between Birmingham International and Euston; meaning if one train breaks down, or the catenary breaks, the entire London to Midlands North of England/Scotland intercity service is royally and totally foobarred.
1) How often does that happen on HS1? Not very.
2) By definition, if HS2 does go down, you have the combination of the ECML, MML, WCML and Chiltern routes to act as backup.