Fossils are not used to bridge the elimination of nuclear, that's already done by renewables. Fossils are used for industry and heat, where nuclear never added anything in the first place. And while technically it's possible to use the heat from nuclear plants to replace the heat-usage from fossils, the German plants were not designed for this, and probably not even in useful locations for this. AFAIK for using heat of plants, they need to be located near the places there it's used. And I kinda doubt many people would like to build nuclear plant in big cities.