I think you could use more citations.

The South Australia blackout was not caused by coal or renewables or lack of interconnectors. If there had been zero interconnects (the local grid was self-sufficient) it wouldn't have happened! The primary cause of the extended blackout was simply a lack of readiness/contingency planning/compliance particularly at several large windfarms. See wiki article.

Coal is generally cheaper for baseload power like an aluminum smelter, even though solar is cheaper during the day. Firming with wind/battery is not yet competitive but is getting very close and is at the point where building new coal power plants is not viable. Refer to gencost report. However, replacing such a large load with brand new renewable capacity is almost certainly more expensive than simply continuing to burn coal, particularly when factoring in the grid upgrades required to make such a transition.