Surely no one is proposing that the US Government would turn off the GPS constellation intentionally. It is not US
Government Policy to turn the GPS off.
That leaves the possibility of loss of signal due to system(s) malfunction.
I will leave it to the experts on how many simultaneous failures are required for a complete loss of signal. However, what is important is what is done if the signal is lost.
Widespread installaion of ADS-B need not result in a reduction of system safety in the event of a loss of GNSS signal. There will still be useable radio navaids and procedural ATC/Directed Traffic Information backed up by TCAS just as there is now.