Dunkelflaute can be dealt with by burning hydrogen, not fossil fuels. You need backup turbines, but a simple cycle combustion turbine power plant is about 20x as cheap to build as a nuclear power plant, per unit of power output. So backing up the entire grid with these "Dunkelflaute turbines" is not expensive compared to the nuclear solution.

Europe has many petawatt hours of potential hydrogen storage capacity in its salt formations.