So if Twilight succeeds and brings about Fray's world, where there is no magic, and all magical or demonic creatures are banished from the Earth... what happens to Dawn?
What happens to Dawn?
Nothing. She's human now.
All the fake memories of her childhood fade, but she remains.
She turns back into glowing green energy.
She's banished from this dimension too.
Something else which I'll explain in comments.
Dawn? Dawn who?