I like that idea. A nuclear wasteland is returning Earth to its natural state.
When you're dealing with geologic timescales, a nuclear winter is just a minor hiccup. You can expect something a meteor impact or a super-volcano eruption every few million years. It's no biggie. The hard part is killing all humans, which I supposed would require channeling the initial deaths to power a genocide spell.
But then aren't humans are just a slightly longer hiccup on the geologic timescale? One that will eventually end?
I'd point out that the "natural state" thing is a human perception and with all intelligent life dead, it would become as meaningless as any other concept, but we are talking about a show with undead, ghosts and witches.