How can thousands, millions of people doing a job be reduced to such a binary statement

The reason why most people (including myself) say ACAB is because of the system of policing, not the merits of any given police officer. Systems are inflexible and adverse to change. Individual good cops can exist, but once again, the system itself is the problem. A good cop can never fix the system, nor could a hundred, or a thousand. A million could, at best, give the illusion of a good system. People often say a rotten apple spoils the bunch, and I think that looking at policing from the perspective of individual rotten cops, or rotten cops “spoiling the bunch” is problematic when the system itself is rotten. And for participating in the system, yes, all cops are bastards.