It *is* okay to say "you're not a teacher" to ex-teachers who are:
a) telling teachers something isn't a problem
b) telling teachers that more effort or compassion will solve a problem
c) claiming to speak for teachers
d) taking power from teachers.

I mean, I think this is obvious, but I keep seeing ex-teachers acting as if any teacher who does this must be claiming that ex-teachers have lost all expertise or that they must immediately lose all respect for all ex-teachers even those who aren't doing this stuff.

There's absolutely no reason we as a profession can't notice that some of those who claim most expertise about how to solve every problem in education, no longer have to solve any problem in education and that some of those who claim to be best at teaching kids, don't.