In the last several Administrations, can you name one time when there wasn’t some sort of investigation going on?
The Obama campaign and Obama himself were never under investigation, neither was H. W. Bush. Until the Iran-Contra scandal, Nixon was the last FBI investigation that involved a president. Then in rapid succession, there were Reagan’s geopolitical money laundering, Clinton’s Whitewater, and Bush’s CIA leak. But none of them fired the FBI director during any of these investigations knowing the optics of such a decision.
All those investigations were finite and they had plenty of time to fire the directors before and after. In fact, Clinton actually fired Sessions for ethical lapses after securing approval from Congressional leaders. At the time, he was not under investigation, and when he was, it wasn’t even discussed as even a remote possibility. You’re not even grasping at straws anymore, you are clutching at the loose fibers.