I wonder what Hickman's original plan was to conclude his x-men run before the x-offices decided to milk his ideas/the new status quo indefinitely.
In any case, i guess they finally got tired of it and want to hit the reset button again.
The most he's said recently is that Moira wasn't going to be the "big bad" and that no one would've guess who the main antagonist was supposed to be. Also, the Phalanx stuff would've been a major plot point that would've been built to and developed in a 12 issue Imperial Guard book that he scrapped once he realized he wasn't going to continue on with the X-Line.
I don't think the X-Office wants to hit the reset button. It seems to me they are forcing this "massacre" so they can have more stakes during this 3-6 month arc and then make Krakoa less crowded and probably without villains once Fall of X is done. If I had to guess, the island will still be a nation, now with a "mutant democracy" and mutants will be welcomed to come and go as they please, and The Five will still do their thing.