simplistically, i understand part of the current need for the existing coal stuff (large steam turbine generators) is that they are currently the "big stick" that "force" all the other generators to synchronise with the coal generated network frequency (when brought on line to add supply or for DC to AC for solar etc).
That is slowly changing as more independent "little" networks can be installed but (simplistically) when all 6 east coast states/territories are linked together the frequencies need to match each other... so there needs to be "something dominant setting the tempo" sort of thing.
Layman here... so laymans understanding.... but a time and money thing to change that is my understanding.
