> Newton's First Law didn't change much in the time that has passed.

No, but technology has moved on quite a bit. Heavy fly wheels are no longer the state of the art here.

A few tens of GW of battery capacity (i.e. a few dozen nuclear plants worth of capacity) that can switch on/off in milliseconds can do a lot for grid stability. That's part of the reason why grid operators are rolling out so much batteries. It's not necessarily about supplying energy for a very long time but about smoothing out peaks and dips in energy supply and demand and responding more or less in real time to that.

This stuff is basically being rolled out at industrial scale in a lot of places. Australia, China, etc. pretty much run increasingly on mostly renewables. This is no longer as speculative as it would have been ten years ago.

Yes, there are engineering challenges with rolling that stuff out in a lot of places. And even more policy and regulation challenges. Actually that is, by far, the #1 challenge in places like the US and Europe. Grid operators are simply structured and incentivized wrong to deal with this stuff efficiently. Texas is actually not doing too bad relative to e.g. California. But they clearly have some challenges still.