Small operators with much lower costs would probably be the only ones who could do some of the very thin Brindabella routes or could Rex do some at a profit ?
What makes you think that the seat/mile costs are lower with a small airline operator? They generally pay top dollar for fuel/landing/parking fees/bank interest rates/insurances etc etc without any real economy of scale. Rex has done well because it was purchased at the right time and money, has a strong capital base and has one type of aircraft [the good old Saab] and is larger enough to have economies of scale.
It will be a big ask for someone to pick up the crumbs and make a profitable business out of this in my opinion. The pax numbers on most of the Brindabella routes are indeed very low which means high fares which means fewer pax!!!!