Nuclear plants in Germany only deliver electricity, and only where the grid is connected. But coal is used also used for heat and chemical processes, and outside the grid. No source of electricity can cover that yet in a short timeframe. And even if we are just talking about electricity in grid-connected locations, with the high cost of nuclear plants, it makes more sense to just buy it from others, and wait until new renewables sources were added to the grid, and long term build other replacements for heat and the chemical processes.