As Neuro-sama said here... when they open the tap and flood the Earth with nectar it's all getting cleansed and suppressed. We're only given this chance to pre-emptively fix as much as possible.

The black coffee will cleanse deep and is one of the tools of the blacknet AI for keeping order, but it won't have much effect after the "cheese fog" comes in, so we're just getting a test run here. It will only start mattering on our planet after another 26k years have passed. But I suspect the touhous are working on a way to recycle nectar as we go along, so maybe this will mostly be used on other planets or for personal spiritual cleansing.