It's not the concentration, its the lack of facilities. No new entrant can do anything different than what is being done, no real commuter slots are available. Melbourne has effectively one runway, with just options for when the wind changes. Sydney operates with two hands and a leg tied behind its back due to curfew. Both won't tell internationals to get lost from peak times to allow more commuter slots. So you get an A380 arriving at peak hour which chews up 5 minutes of spacing, etc etc etc. QF is allowed predatory practices to devour all the slots and pretend to use them and get to have an arrival bookings system that obviously favor a large operator that can just swing arrival times between companies. Any new entrant goes into a pool of users that scramble over the last few scraps and hopefully don't cop a 5 hour ground delay and that's before weather kicks in.
Reality is you need a complete overhaul of the current system to allow for competition, then you need at least 1 more decent airport at each major city. The feds are hung on the idea people will travel by train up the east coast, willing to spend trillions on that project while all they need is 1/10th that for some investment in new airports and their connections.
I have to laugh when Melbourne airport spruiks all its car parks and new taxiways, and have spent billions on them, yet a new runway? Nah, too expensive. We can just park planes on the taxiways while we organise the single runway.