GPS is comically easy to jam or spoof and assuming it will work all the time is just a silly, dangerous assumption.
We just happened to live through a brief period where you could believe that
Annoyingly all of these systems have VOR and DME equipment, it would be trivial to have a back up position to well within en-route navigation accuracy limits and certainly close enough to prevent this kind of accident. But regulation and certification requirements being what they are they are no one does it.
In an ideal world we'd have some kind of INS but the ring laser gyros and accelerometers required are stubbornly expensive.
However, that doesn't change the fact that the intersection that ATC are flying you towards are actually now defined as coordinates, not radials or distances from nearby VORs.
If an ATC tells you "direct OKSAW" they won't give you a VOR radial to intercept - you are expected to go straight there in a relatively straight line from your current position. There were boxes like the KNS80 that could do VOR-VOR navigation to do this kind of thing but it's not user friendly by modern standards.