It is an unfortunate truth that ground-based navaids are expensive to run (power requirements), and repair (imagine the cost of getting 2 technicians to Birdsville or Balgo).
With our industry continually pushing for efficiencies, those talking loudest get heard the most. This translates to "those paying AsA the most money" (i.e. QF, DJ, JQ etc).
Ground based aids at Birdsville or Balgo are of no use to them, so they naturally baulk at paying for them. For that matter, aids at Ballarat, Port Augusta, Casino and Kiingaroy are of little or no use to them either.
This raises the question - who will pay?
Unfortunately the extinction of NDB's and VOR's is only a matter of time. In PNG now, I think all except 2 or 3 NDB's are gone.
If you are considering buying an aircraft now, forget about an ADF; make sure it has two 530's.