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.
You are 100% right this is the heart of the problem and the biggest issue facing Bonza and the industry as a whole. Until there are two airports in Sydney, Melbourne, Brisbane and Perth there is never going to be any real competition in aviation. Throughout the world all the real LCCs fly to secondary airports some of which are not even close to the city they claim they're flying to. As it stands the second airport in Sydney will just add congestion to the Melbourne Airport as they won't be able to handle any increase in traffic.