Isn't Griffith trying to limit the power of the benevolent magic while also trying to move the physical world into a state that also greatly increases the power of magic? It seems like that's a world that constantly in a state of fucked with the cast taking over roles of people in the past. Schierke is becoming her teacher, Guts is becoming Skull Knight and Farnese is becoming former Schierke if that even makes any sense.
We have no idea what in the fuck Griffith is really trying to do. I don't think Schrieke's magic is benevolent, it's just not actively malicious like that of the Idea of Evil's. It's nature magic, and I would actually think it'd get stronger rather than weaker from Griffith merging the two worlds, but who knows.