User blog comment:BlackWizardZeref/Fairy Tail Chapter 384: Attack of the Celestials Review/@comment-7418318-20140516225538/@comment-7418318-20140516233306

The chance of anyone having the necessary bonds of trust, the necessary power and being in a dire enough situation to go through with destroying the key are rather low, so it's quite possible that no one has done this before and Aquarius just knew of this in theory without ever having witnessed it.

But I don't think Aquarius could be so easily replaced. The Ecliptic Zodics aren't just a group that was picked at random. As the name suggests, they all share a certain relative place to each other in the night sky.

Unless the Celestial Spirit King can move entire star-systems around at will or "uncouple" a Spirit from it's constellations, Aquarius will remain tied to her constellation and can't be replaced.