That's a pretty big question. In short nothing stops an operator with the appropriate AOC and requisite structure in place to satisfy the relevant authority from operating on or even opening up new routes. You either want to make money or perhaps compete with the other operator for whatever reason and cop a loss. Sometimes some regional routes are given a subsidy by the relevant state government, as you find in QLD, meaning they are viable to operate on.
Your question would be best answered by an accountant though, they seem to run airlines these days.......