Geoff, based on your previous thread creations, anybody would think you have a vendetta against our military and national interests. It shouldn't require much explanation, however clearly you need it. Airspace must be designated as PRDs at times, often for our military largely to ensure it remains effective and capable, but equally to ensure the safety of uninvolved aircraft. They almost always deactivated when not in use.
What happened in the SCS is very different, China built up submerged ground so that it ended up above sea level, and then tried to claim it as legitimate territory that should be afforded 12nm sovereign buffers, impacting freedom of navigation. This is clearly in contravention of international law. Are you proposing other airspace Australia control's (e.g. FIRs) outside 12nm's is 'illegitimate', or just restricted area's? They are no different in concept - just application. Every other nation in the world imposes rightfully their authority to control aircrafts movements in their airspace through active management (e.g. ATC flow management) or latent airspace procedures such as restricted areas. Your issue with these PRDs comes across as seated in a disdain for the purpose of them, not their legitimacy.