Q: what is the cure to the Rash?
A: nothing, there was never going to be a cure found. The only cure is a bullet to the head.
That's quite... depressing. But it does make sense, because the moment a cure would be found, the world could start healing itself and the story would end there. Of course, that process alone would take years (even decades) to accomplish and could (or rather
would) be worthy of its own story in SSSS canon...
But yeah, I can also understand the idea of not providing an instant fix to everything. For one, like I said before, it would end the story as it has been thus far (or rather, force it into certain direction compared to not having the cure, which is far less restricting). And two, it would've been quite cliche.
I DID hope that there wouldn't be an insta cure available at the end of adventure 2, because that would've been some Disney movie level of
happy ending. Rushed, sloppy and indeed very cliche. But the idea that there was NEVER going to be one at all is another thing entirely.
Of course, there's always room for speculation; *clears her throat, digs up her roll of tinfoil, folds it neatly into a boat shaped hat and then puts it on*: it's always possible, that there USED to be some kind of an idea for the eventual cure, before Minna scrapped that idea entirely at some point. The same way any further SSSS stories got scrapped here.
IIRC, didn't the experimental vaccine in ADV 1 technically stop people from mutating into trolls? I have this memory in my head about Mikkel saying something about "different bone structure" on the bodies that had used the vaccine. The vaccine couldn't save their lives, got their souls stuck in a limbo but, IIRC, it DID stop the mutation, right?
Of course, that vaccine itself could've always been meant as a mere plot devise to explain the murder ghosts, nothing more. But the idea that that vaccine was never even CONSIDERED to be something that would've been refined in later stories (or alternatively, some other vaccine from elsewhere)... Well, perhaps that was indeed the case. Or perhaps it was considered as an option at some point, before getting ultimately scrapped, in order to keep the SSSS world's status quo as is for all eternity ¯\_(ツ)_/¯.