I read the GP as saying that SA GPS was originally made available as a result of the KAL007 crash, so presumably they didn’t have access to GPS at all on the flight? Then much later the SA restriction was removed, unrelated to a plane crash.
Though KAL007 deviated from course not because of some INS fault but just because autopilot didn't switch into INS following mode at all. Most likely crew just forgot to flip the switch, so GPS availability wouldn't have helped anyway.
Being out in the middle of the ocean with only INS supposedly keeping you on track is different than a GPS receiver that you can look at and see that you are not where you should be. Obviously the same thing would happen if no one was paying any mind to the cockpit instruments but at least a GPS receiver offers a solid reference to compare against. I really can't imagine how nerve wracking it would be trying to fly a plane without INS or GPS over an ocean. Your navigator would do their best to keep track of location but you'd probably end up way off course by the time you saw land.