Why can’t the FBI be shut down entirely?
Exactly. They have or could I say, the world, has worn out their usefulness. Other agencies conduct criminal investigations that are specific to that agency. No need to have a Jack of All Trades agency when there are dozens doing the same job and doing it better in most cases. As for internal threats to the homeland, via terrorism, even though it was only supposed to be a place to, deconflict/share information, DHS now has it’s own agents, so they can handle that job.
Contrary to popular belief, the FBI is far from the most virtuous group of individuals on the planet. It has a very long and sordid history, just never hear much about it because they’re always portrayed as riding in on their trusted stallion to save the day. When that is far from the case.