Longer term you're right. But in the short term it's a big project over decades to roll out enough renewables and storage to allow a 100% renewables system.
Snowy II will provide a lot of what's required both in storage flexibility and renewables energy. Batteries will become cheap enough down the track. There are other possibilities like Bass Link expansion, and hence increased utilisation of Tasmania hydro and other renewables that can be developed there. And some transmission line capacity build will likely be needed to strongly interconnect diverse renewables the length of the East Coast. There are some interesting projects looking at DC link submarine cable inter-ties to Indonesia from massive solar farms in northern WA, that would increase the diversity and export value of increasing renewables. All that's been discussed on these threads before.
My best assessment, from knowledge of the industry, but without being close to an actual planning and modelling group, is that over the next 10 years you can build as much solar and wind as you can and, with a bit of care, be fine with coal and gas firming that - provided that the system operator is given the tools to manage that increasing renewables mix with coal and gas firming. And if that means keeping one or two coal stations going in that firming roll, to extend life rather than see them decommissioned, then that may be a valid decision.
South Australia largely was a problem of transmission lines falling over, not a renewables issue. But it does point to the importance of strong interconnection.
After that you are likely going to need additional storage solutions (beyond Snowy II) to balance increasing renewables to 100%.
It looks a pretty smooth transition, so long as all your coal doesn't decommission while you still need them for firming. That's a risk given declining utilisation of coal as wind and solar pushes them off grid much of the time. For a period you are likely to need some capacity availability contracts to pay coal generators to be available when needed.
That won't be a perfect view of the future, but pretty indicative I think.
