I don’t know of a reason they could become incapable in the sense of being essentially deprived of this priestly power (is there a scholastic term for this?). Incapable due to a persistent defect of intention that isn’t filled by the Novus Ordo form, because of an absence of belief in or knowledge of what the mass is, now that I could understand. If you have some apostate saying masses he believes to be mere re-enactments of the Last Supper as a meal - how could that be valid? But if he really wanted to offer the sacrifice of Calvary to the Father in the transubstatiated body and blood, with proper form, what would prevent him?
Anything else I could think of would be walking the knife’s edge of Donatism.