You’re confusing definition with inference.

Of course "a felon has previously committed a felony" is tautological. But "a person who has committed a felony is substantially more likely to commit serious crimes in the future" isn't.

It's a well-established empirical claim, and that's why they're prohibited persons.

As for magically predicting whether someone will become a prohibited person: we can’t, and even if we could, that would mean preemptively restricting a constitutional right.