I feel like gps nav software should start considering the jamming and spoofing cases as real conditions they will encounter that should throw up an INOP warning.

It wouldn't take much smarts for it to consider the type of vessel it's in and the max and current speeds. If the calculated position of a 10 knot vessel suddenly jumps 100 km away in seconds, that sure seems like a spoof. And if you had 10 satellites locked and then you have none, that sure seems like jamming, or you're in a tunnel; INOP either way.