So, Rika’s prophecy is still giving me trouble here, but maybe something else will come to mind if look at some of the other mysteries to tie the theorizing together.
Why does the GHD not occur in the Sonozaki rampage arcs?
Well, the presented Fantasy solution would have us believe that with Rika killed, Takano couldn’t justify wiping out the village under the emergency manual scenario. I think there’s something to the idea that Shion’s victims being removed from play prevented the GHD, but just because Rika is a major character doesn’t mean she is the only one that matters here. In addition to Rika and Satoko, Shion also kills her grandmother Sonozaki Oryo and Kimiyoshi Kiichiro.
So, I feel like those deaths may be the important ones here. If we suppose that in the anti-fantasy version of events, the motive is to get at a uranium deposit, then the GHD might be the ‘bad end’ of a complete negotiation break-down behind the scenes. Without the heads of houses to negotiate with, the negotiations are put on hold while the secret conspirators figure out who they need to talk to for the negotiations.