A biogas turbine for the last few percent of demand??
Peak electricity demand occurs when solar generation has dropped to nearly zero. It's not the last few percent, it's the last 90 something percent demand. This is the entire problem with solar (and wind, though slightly different patterns).
If you're going to "they should just" it, you really need to know at least that. It's just a hard problem even in Australia. The reason these "dirt cheap renewables" have not been pushing electricity prices down to historic lows anywhere in the world is that you've kind of been had by the marketing.
Solar and wind are very important and very cheap where applicable, and with more storage, better grids, and consumers that are better adapted to them they should gradually improve. But they are not going to "just" anything.
Remember how wind and solar was so cheap that it had already killed coal? That was a common mantra I heard maybe 20 years ago. Since then solar panels and wind turbines have become even cheaper and better so surely they must be moving on to just about killing off natural gas too... But no, it turns out 60% of Australia's electricity is generated by fossil fuels today, 40% being coal which is twice the amount that solar generated. How could that be possible decades after coal had been killed by solar? Really was some pretty wild propaganda.
This is not due to government corruption and incompetence and a cabal of coal barons preventing renewables. Nuclear maybe, but solar no. There is electricity generation surplus when solar is working in Australia, they turn off wind turbines and solar panels and try to give the electricity away for free. Cost of solar panels being zero would make approximately no difference to those 20% solar and 60% fossil fuels numbers.