This was true from it's earliest days when it was pulling dirty tricks on civil rights and anti war protestors.
Thinking this kind of things is cool when it's used against your political opponents is incredibly short sighted.
That's literally it. As long as the Feds were just cracking the heads of black protestors and snatching brown people up out of their beds, Trumpists were hard-core thin blue line. But the Feds dared disturb their oshi and now the same Trumpists have gone full #ACAB
It's basic fascist ideology - loyalty to the leader above all, and belief that the leader is above the system.
The FBI are investigators, not traditional police; they aren't generally the ones people are talking about when they say "thin blue line". It wasn't the FBI cracking heads and "snatching brown people out of their beds." The phrase is a reaction to perceptions of widespread civic disorder and violent crime; it really has nothing to do with the FBI at all.
It would be logistically and all around easier to replace the corrupt FBI and it's culture than these two attempts by entire nations at replacing their corrupt police forces.
It is needed. Not because of anything that's happened the last couple years, but because the FBI has been run and staffed by criminals for decades.