What d'you mean? The Dryad sells Purification Powder in pre-Hardmode already, so I'm not sure what you're saying. >_>
I meant a more final solution I guess. To me, using Purification Powder is more so just trying to push against it. When The Hallows comes into existence, you finally have a way to fight back. Overtake the dark land and replace it with light as opposed to trying to push it off a bit.
That's just me though.