Whether or not Mueller would be replaced if ousted is unclear. Rosenstein chose to bring in special counsel and could seemingly decide it's not necessary to replace Mueller. After all, Trump's alleged issue isn't with Mueller, but with the presence of special counsel in general. Democrats in Congress would certainly call for the position to be filled again, but their powers are limited here. Congress only has the ability to request special counsel; it cannot appoint anyone on its own. However, the congressional investigation into Russia's interference in the 2016 election would not be affected if Mueller was fired.