This is what ACAB means btw. It's not that good people can't become cops, it's that they can't stay good, or can't stay cops. They either get pressured and eventually broken into habitual complicity or just forced out entirely.
The dynamic is real and because of it the statement is nearly always true, ie a useful model for predicting outcomes in the world.
Cops can quit and do something else at any time. It's not an identity or protected class aside from qualified immunity.
ACAB.