From the very beginning of the Kingdom Hearts series, the return of Kingdom Hearts was treated as a good thing.  In the story that Kairi’s Grandma told her, she said, “But the true light still sleeps, deep within the darkness.  That’s why the worlds are still scattered, divided from each other.  But someday, a door to the innermost darkness will open.  And the true light will return.”

However, somehow it started being treated as a bad thing by Kingdom Hearts III.  Before the fight with Ansem, Xemnas, and Young Xehanort, Riku said to Master Xehanort, “If you do summon Kingdom Hearts, we will defeat you, and we will close it again.”  And that is exactly what they did.

So, why this change?  Why is the return of Kingdom Hearts suddenly treated as a bad thing?  I have an idea as to why that is.

I think that in order for Kingdom Hearts to reunite all worlds, it was going to have to purge them, which is what Master Xehanort wanted.  Everyone and everything would be consumed by darkness and destroyed just so the World could be returned to the way it used to be: one single united world that was equal in light and darkness.

Therefore, even though many have thought that Kingdom Hearts' return would be a good thing, such as Kairi’s Grandma, they did not realize that it would actually be a bad thing.

Edited February 3, 20205 yr by Dagesh Lene

This creates a very interesting paradox. Because if KH is opened, then basically the world goes through a Rapture level event and then resets. That would have made everything in the series be for nothing, honestly.

But perhaps there's more that we don't know, and that may be what the MoM is after! Maybe what's in the Box is key to what will transpire with Kingdom Hearts itself! :o