That's what ACAB means though. You cannot trust cops, because there's no real accountability for them. Why is there no accountability? Because their colleagues lie for them, their bosses lie for them, the prosecutors decline to prosecute them, judges trust them implicitly, their unions intimidate mayors and lobby politicians for more funding, tougher laws (for non-cops) and less accountability for themselves.
The system is so fucked up that reforming it seems like a waste of time. Actual "good cops" get squeezed out or worse. You might as well assume that ACAB, because the stakes are too high to assume otherwise.