World could be destroyed by a Nuclear Holocaust between leaving New York and arriving overhead London, and the INS would calmly continue to navigate blissfully unaware !! Happy Days.
This (the Americans turning off GPS) has for the most part been used for a decade or more to scare pilots from using GPS, and has been a part of the CAA campaign to keep a lid on GPS approach introduction.
What everybody pushing this line doesn't realise is that if this actually happened (WW3, basically) we would not be flying. GA would be banned on day 1 of any large scale hostilities - as happened in WW2 for example.
And if your airliner happened to be airborne on the N Atlantic route when the world comes to an end, you only have to fly a heading within about 10 degrees to find Europe
And your INS won't help you land on an IAP - far too inaccurate. You would have to descend over the sea and continue visually below cloud.