Those investigations would go on even if Mueller leaves. When the Justice Department initiates an investigation, it can’t be closed without following a set of procedures that ensure cases aren’t shut down for improper reasons. If a case is opened, it can’t be “declined” — closed without bringing charges — without a detailed justification for closing the case. As a former federal prosecutor, I’ve declined my share of cases, and it takes time. Declining even a routine case requires a written explanation justifying the declination, citing specific reasons that are consistent with Justice Department guidelines. In more complex or high-profile matters, much more extensive memorandums are prepared. Once in my career, I inherited a complex case that another prosecutor unsuccessfully sought to decline, and I ultimately charged the case.
Any cases Mueller’s team is working on wouldn’t magically decline themselves if Mueller is fired. The reports of interviews in the FBI computer system wouldn’t delete themselves. The documents and other evidence collected by Mueller’s team of FBI agents and prosecutors wouldn’t destroy themselves, either. In fact, Justice Department procedure is to retain evidence for years even after a file is closed. Whenever I closed a file, I had to specify how many years the evidence would be retained.