South Australian electricity is actually some of the most expensive in the world due to incredibly high transmission costs.
The Australian Energy Minister, Mr Frydenberg has even placed some of that blame on the SA Government's and it's "self-inflicted wounds" on deciding to transition away from coal to clean energy.
However, in the article above has there is a link to this page:
https://reneweconomy.com.au/south-australia-had-lowest-cost-...
which states:
The renewable energy state of South Australia recorded the lowest prices in Australia’s main grid in October, a month when it sourced well over half of its it's electricity supply from wind and solar.
That would suggests that transition to clean energy is working out just fine.
The reason people don't do this is that the cost of a grid interconnect (the paperwork, approvals, etc.) outweighs the profit from the very short periods of time the price really is negative.
In this case, demand is lower than supply, so either you decrease the supply (shutting down the solar farm here), or increase the demand (lower the price). Because this is transient, and some powerplants (e.g. nuclear) take time and effort to shut down, for those operators it is cheaper to pay you to use their energy during those times (negative prices), than to shutdown/restart when needed.
Another way to think about it: uses like those you propose that can soak up excess energy in the grid are exactly why energy markets are allowed to float, and even to go negative. If there's a surplus, they want you to use it, and if it were economical to store it and sell it back to the power company later, both they and you would benefit. That this isn't common suggests that it isn't economical.