The question was, "So why is it presumed that the VOR system cannot fail?",
not, "So why is it presumed that a VOR cannot fail?" The loss of a GPS
satellite would not render the GPS system unusable.

.... and my response addresses exactly that. The failure of a single VOR
(or even twenty) won't cripple the VOR system. But the (albeit
unlikely) failure of twenty satellites will cripple the GPS system.
There is more systemic redundancy in the VOR system, just by virtue of
there being more VORs, and by virtue of the fact that a single VOR
affects a relatively small area.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.