>If your nuclear capacity can cover the worst case where solar and wind aren’t producing, it can cover all cases, and the solar and wind become irrelevant.

This is plainly false; nation-wide energy consumption isn't fixed, it adapts to supply. Not all usage of energy is equally valuable, if the prices go up, you might choose to turn off your TV, but you'll probably keep your fridge running. With more energy, more activity can happen at lower prices.