Simmer down guys these are just silly acronyms. To cool you down further remember that Apollo misions to the moon used a single INS with the attendance gimbal lock problems and all. Today FMSs backed by two or three independents INSs are the norm and if these 3 units fail, so what, you still have GPS, LORAN, VOR/DME in triplicate and ATC. Getting lost is not an acceptable excuse for not getting there any more.
What ever happened to the good old navigator? Let me see what were they known as ....