Case in point is a very cloudy day with no wind. Depending on where it is demand could be very high if it's cold out and heaters are running full blast. You will essentially need a full load supply to come from somewhere or you'll get rolling blackouts. Some regions can compensate with importing or hydro but otherwise you would have to power up the coal and gas. Batteries could maybe work in the future but again this would drastically increase the cost.
Flow batteries dramatically change the cost calculations for energy storage, and are here now.
Additionally, most areas are increasingly moving towards decentralised generation which changes the calculations too.
I think it's perfectly reasonable to question those solutions, but an equal amount of traditional power generation as backup seems excessive.