There's also the fact that Trump can fire Mueller at any time and there isn't anything FFXIAH users can do about it !
The left is living in an alternate reality where the separation of powers doesn't exist. The DoJ reports to the head of the executive, which is the President, Mueller works for the DoJ. Mueller is not an extra-judicial wing of the Democrat party, despite how much people like Chanti and Vic pretend it to be.
The proper way for something like this to be done is Congress starts it's own investigation, which they have the Constitutional power to do. That investigation would interview people, collect facts and ultimately present it's finding to the body (House or Senate) that ordered it, that body would then either take action or not. This is how the Constitution provides over watch of the Executive.
The issue with that is such an investigation would be limited in scope by nature, it wouldn't be a witch hunt or a fishing expedition.