Because users can now manually update these values in pg_class, there wouldn't be a way to detect the difference between a bogus relallfrozen value due to VM corruption or a bogus value due to manual statistics intervention.
Er..you had me until this. If manual monkeying of the system catalogs leads to a "bogus" error that resembles a real one, then sow the wind, and reap the whirlwind. I don't think that should be a consideration here.