The only reason Class E works is the big sky theory.
No, the only reason Class G works for IFR is the big sky theory.
Class E in Australia has been implemented (deliberately or accidentally) to provide no benefit to anyone.
Class E is supposed to replace Class G, on the principle that aircraft operating in IMC should have ATC assistance to avoid each other. It is not intended as a replacement for Class C/D*.
When the decision was made that Class E will be implemented WITHOUT providing additional service to IFR, the point of the project disappeared. After that it became a process of finding places on the chart to put a bit of Class E, so someone can tick their "Class E Implementation" box.
*Although once you have a choice between C/D/E/G instead of just C/D/G you will probably find airspace that is class C only because it can't be G, which might be changed to E.