(I acknowledge the following is approximate for numbers, but the principle is verifiable).

I wish I could remember the source (may have been annual Air Force Instrument Refresher Class; I learned this in the last 12 months), but the GPS Constellation requires about 24 Satellites to give 3-D coverage (ie, 4-5 satellites viewable) at every spot on/above earth.

A total of about 30, maybe 32 had been placed in orbit. I think 26 or 27 were still operational. Two or three "spares" above minimum.

Some of those in operation are first-gen satellites. They had an expected service life of 8-12 years. Some of them are over 16 years old.

Most of the satellites in orbit are of a 2nd and 3rd gen variety. They have a longer service life, maybe 16 years. None have been exceeded, although one or two of the new satellites are dead.

Here is the kicker:

Based on expected attrition, there will be fewer satellites in the constellation than the required minimum around 2017.

Currently, there are no plans [due to no money] to launch any replacements.

The FAA views this as "We can shut down OMs, LOMs, NDBs, and VORs because the GPS system is superior, which will save the FAA money. The FAA doesn't have to worry about GPS funding---that's the military's job."

(But the military doesn't have the money to launch new satellites).

How expensive is a new satellite, and how much does it cost to maintain an existing VOR? I don't know, but I'd guess you could maintain 100-200 VORs for the price of one satellite.