That's a funny thing because, as with all absolutes, it's trivially easy to prove it wrong. All you need is _one_ cop to not be a bastard to prove it wrong.
I've known several non-bastard cops.
If you work alongside bastards, like civil-rights-violating bastards not chew-with-your-mouth-open bastards, and aren’t actively working to get them removed from the force - i’ve got bad news about your bastard status.
Which is the point of the saying. It’s not that all cops are individually bastardly, it’s that all cops are part of a system that both protects bastards regularly, and does systemically bastardly things (like say heavily policing crimes of poverty while ignoring crimes of wealth).
> trivially easy to prove it wrong
Only if the other person is arguing in good faith and believes you.
There's another relevant saying about apples you should check out.
Then you, like many others, misunderstand what the saying All Cops Are Bastards means. It's not an observation about the morality of each individual cop. It's shorthand for the fundamental corruption and injustice inherent to the institution of policing itself.
If Mother Theresa or Mister Rogers becomes a cop, ACAB isn't suddenly disproved, because it's not about specific individuals and their specific moral qualities. It's about systemic and fundamental problems with policing as a whole.
See how ACAB is a lot easier to say?