If Trump wants to do this, I say let him. It might be the sort of thing that could backfire enough for Republicans to support impeaching him.
And if Hillary had won and had purged the FBI leadership because she was unhappy with their actions during the election would that have merited impeachment? Or would that have been different?
Impeachment doesn't happen because of a specific wrongdoing. Nixon's involvement in the Watergate cover up was far from the worst thing that recent Presidents have done. Nixon was forced to resign because powerful interests in both parties wanted him replaced as President and found (or manufactured) the pretext for doing so.
