Following on from this question: Can I True Polymorph a goblin into adamantine (and then forge him into a sword?)
Casting True Polymorph on an unwilling creature (relevant parts included only):
You transform the creature into a different creature...or into an object.The transformation lasts for the duration, or until the target drops to O hit points or dies. If you concentrate on this spell for the full hour, the transformation becomes permanent. An unwilling creature can make a Wisdom saving throw, and if it succeeds, it isn't affected by this spell.
Creature into Creature: The target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to O hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to O hit points, it isn't knocked unconscious.
Creature into Object: If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form. The creature's statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.
The consensus seems to be that if the creature fails the Wisdom save, you can transform it into a weak creature like a slug or an object like a cup, and can then wait for 61 minutes concentrating on the spell, then stamp on the item and kill it, permanently, because that was its new permanent form. But this seems terribly unbalanced, requiring a single wisdom save to be failed (and these can be influenced with features such as Heightened Spell and Magical Ambush) to kill any creature which can be polymorphed, no matter its stats. This also seems to make True Polymorph a better version of Power Word Kill (if delayed and requiring a save) since it is completely uncapped by health, only by Wisdom save proficiency, as well as including many other functions. This makes it seem unlikely this is the correct interpretation of True Polymorph since it renders another (already weak, for that level) spell almost entirely pointless.
I presumed the spell to be saying that after one hour of concentrating on it, the polymorphed form no longer needs to be concentrated on and could last forever, but will still revert back to the original form if reduced to 0 HP, as described in the linked question where the creature (polymorphed into crafting materials) was cut apart. This seems to make much more sense and retains balance.
Which interpretation is correct? Does the 'permanent' section overrule the 'transforms back if reduced to 0 hit points' part?