Let's say you already have controllable nuclear power and you want to switch while maintaining capacity, best options seems to be either storage + renewable or fossil. Actual production on a given period is an orthogonal problem, we are discussing controllable capacity because you don't want population and industry to suffer and you plan carefully.

Now replacing nuclear capacity, with whatever else, in order to reduce total emissions doesn't make sense, you are doing this for a totally different purpose. Replacing nuclear capacity by investing in new fossil capacity, in order to reduce total emissions, doesn't sound that smart.