No, planned shutdowns due to refueling or maintenance are technological constraints, and so are hutdowns due to accidents. And it is a stretch to talk about provision "on demand" in thwe case of baseload plants: baseload plants are good for providing constant power, but demand isn't constant.

So, back to the point, all plants are intermittent, and at least those normally run at the maximum power possible (nuclear, "brown coal", wind, solar, tide, wave) require balancing capacity.

_{*Lunatic*, n.
One whose delusions are out of fashion.}