Most ACCs I know of have an emergency surveillance system that is approved as fall back to empty the airspace and provide limited service for limited periods of time. (Some HOSP/SAR/MIL, as well as catering for planned downtime of main systems. Think software upgrade or similar)
And for those that get all fired up about there not being a procedural fallback: As ATC Watchers says, its all about money, and NO ONE is willing to pay for any of that. And to be real, there is really no need, the fallbacks that are in place are adequate.