What a debacle.
Any truth to the rumor that the report was delayed due to disagreements about whether there should be more discussion on the airspace category?
Self separating 5 aircraft inbound by radio is difficult in VMC. In IMC it would be extremely difficult - separation becomes more luck than good management.
This is exactly the situation Class E airspace is designed to avoid. Instead of 5 pilots with 5 plans developed on the fly for how they will maintain separation, you have ATC with well planned and designed procedures. This relieves pilots of the separation workload, allowing them to concentrate on the actual approach. The report touches on workload, without any real discussion on why the workload is so high.
Without Class E Australia is basically relying on low traffic densities to ensure that you don't get multiple IFR aircraft trying to do the same thing at the same time. Every so often, that doesn't work out. According to the report CASA calculated the collision risk as 1 in 7200 years - I would be interested to see the assumptions behind that.