Why does INS need DME? Why can't it get position fixes off GPS? GPS is already there (and I guess Galileo will be one day for those who think you daren't rely on non-European technology).
New jets etc use both DME and GPS for fixing-up INS errors.
My guess is that DME will remain for this purpose, for ever. I went to a Eurocontrol presentation in 2008 where they said they plan to roll out 300 new DME beacons over Europe, to guard against GPS signal loss / jamming.
Galileo has no relevance because in any scenario where Navstar is turned off, Galileo will be turned off too.
NDBs have all but disappeared in the US of course, and no new aircraft have ADF.
They still need one fitted to fly in Europe under IFR legally; same with a DME.
There are many NDB approaches in Europe, or approaches which use NDBs as a part of an approach.
Enroute NDBs can go tomorrow...