Specific Beats General - A permanently polymorphed creature will stay morphed if it dies
In 5e, you always apply a general rule unless there is a specific exception. That is exactly what is happening in this case. Generally, the target will revert after the full duration, upon the target reaching 0 hp, or upon dying. However, there is a specific exception for when the caster concentrated on the spell for the whole duration. In that case, the change lasts until dispelled, full stop. The first sentence no longer applies in this case, because the second is an exception to it. As a result even if the conditions in the first sentence are met, the creature stays transformed because of the second sentence.
"Until the target drops to 0 hit points..." isn't enough because of effects that can instantly kill a creature without affecting hit points. These effects are usually high level magic like the 9th level Power Word: Kill and the 7th level Divine Word both of which can instantly kill a creature without changing their hit points at all, as long as the creature's current hit points are within a certain threshold. In these cases, you do not drop to 0 hp; instead you just immediately die.