In this case, a CAT3 ILS in Adelaide is unlikely to have made any difference. If the cloud/vis is below the alternate criteria (typically 400'/1600m for airports with low vis. installations) and they had the fuel, the crew would have been obliged to divert. So, it's highly probable they would have ended up in Mildura anyway.
I agree they're long overdue in this part of the world but they're not the only piece in this jigsaw puzzle.