Is that true? Surely things regress to a stable mean most of the time of a few archetypes, no?

Yes. It’s generally stable in most localities. “Instabilities” in this system are browns outs power failures and other events. There are stabilising features within most electricity grids, but they can only cope so much. In general forward planning is down so the amount of dynamic adjustment needed is within allowable range.

But to be honest i don’t know how modern grids have adapted with many more micro generators than in the old days.