I know some of the ones being retired in AUS were on military bases that are being closed down. This is causing some grief with local airports that used them for nav as well.
It is not all that simple, the navaids are getting very expensive to maintain, and development around the airports causes too much interference. Many just have to be retired because they just cant tune out the interference anymore...
Air traffic is increasing, and with a tight queue, the surface navaids get disrupted as well.
GPS overlays are just that, and overlay of the existing ILS or ndb procedure, that provides waypoints for navigation. They do not require a navaid.
Overlays are a great way to get a procedure approved through environment, as technically, the procedure and corridor are already approved. I know that 'T' configurations were designed for all runway ends at YMML, but in operation, the tracks were too varied, and there were a lot of noise complaints, so on 16 and 27, they went back to straight in RNAV GPS from the NDB location.
(the private RNP procedures have waypoints on the ndb location)
The RNP AR tracks for 16 and 27 are still on operation by Qantas.
The cost to maintain and calibrate a VOR or NDB is about $50K/year for each runway end.
A GBAS system is around $2 million to install for an entire airport with up to 26 runway ends....