Larry in TN
En-Route
You are only looking at this from a GA perspective. The FAA looks at it from a national airspace system perspective and most of the users of the NAS have a backup to GPS--DME/DME/IRU.OK....but if GPS goes out for everyone and nobody else in the air has a backup, how many aircraft does ATC have the bandwidth to support?
The minimum VOR network is designed to support DME/DME/IRU.