In the time before renewables all of the rate-of-change limitations came from sudden increases or decreases in consumption, never from the supply side and this has subtle implications for the structure of the power grid and how much power companies can do to control the match between the two.

Think of it as a person walking across a rope with a balancing pole: you can overbalance for a short while because you can correct for that with the mass of the pole. But if the overbalance is too large for the mass of the pole then you will inevitably fall (brownout, blackout).