Why would a VOR be decommissioned leaving the DME?
Many older Air Transport Airplanes have no GPS. I'm a pilot since 1970. I operated A330 for 5 years until 2000 without GPS. The average IRS position (Inertial Reference Systems) in the FMS was updated by DME/DME. On older airplanes we selected the DME Stations manually preferably with a 90 degree cut-angle to obtain an accurate update. A single VOR/DME station could be used for update as well but it's a lot less accurate. Example, if you are 60 NM away from a VOR, a 1° error in azimuth would cause an error of about 1 NM. All Airbus aircraft have an approval for RNAV Approaches down to an accuracy of 0,3 NM with 2 DME stations available. This is why DME's are still useful, many ILS have a DME. Conventional VORs are terribly inaccurate, Doppler VORs are much better even down to a lower altitude, but requires more space.