As part of the MON, the FAA will provide at least one "safe" ILS procedure without any dependency on DME, GPS, or Radar wihtin 100 NM of any place in the NAS where the MON is removing VOR's.
Actually I believe the intent is to have a non-GPS approach within 100 NM. This includes not only ILS approaches but also LOC and yes even VOR approaches.
Anyway, think of of VOR MON as "
Phase I" in the overall VOR decommissioning process.
MON will reduce the number of VORs by up to 50%, as mentioned. The timing keeps on slipping, but let's say this is completed by 2030 for discussion's sake.
But what happens after MON? VOR decommissioning won't stop!
MON is a required "stepping stone" because we need to provide backup to GPS/GNSS in case of a global or local outage (e.g., interference, jamming, etc.) But MON cannot meet "NextGen" level of performance required for PBN, so the FAA, partners & industry are working on an alternative navigation system (APNT: "Alternate Position, Navigation, Timing").
APNT will likely based on a network of DMEs. That is, improving the tech behind today's DME/DME/IRU RNAV, so even if all GPS signals are lost, enroute RNAV can still be maintained to PBN requirements (e.g., RNP 0.3).
For this to happen, the existing DME network has to be enhanced to provide better accuracy and/or augmented with "pseudo-satellite" signals from ground-based transmitters (GBTs). Think of GPS signals sent via ground antennas instead of from space.
(That is why we never hear of DME decommissioning -- the FAA has big plans for them).
The idea is to have APNT up and running by the end of MON (in our discussion, 2030). Then
Phase II can start: most of the remaining VORs, except few critical ones, can then be dismantled in favor of APNT.
(By the way the FAA is currently aiming to have APNT fully operational by 2025).