I'm really sorry about changing Marisa's line here. What was here before was brilliant. For the sake of prosperity, I'm putting it here, where more people will see it than if it was only in the note history:

In that case...
It would have come to this world.
So that's why you came here, right?

The original text isn't really incompatible with the idea that human sacrifice is foreign to Gensokyo, and arrived because that's what things forgotten in the outside world do, but the more obvious implication of Marisa's actual line is that human sacrifice is normal everywhere, and it's the outside world that's weird for getting rid of it.