At first Athanasius’ view of human nature as either always moving towards God or falling away from Him into non-existence struck me as odd, but the more I think about it, this does seem to be the logical consequence of creation ex nihilio.
If we really did come from nothing, then it makes sense that we can only remain something if we have some sort of inertia. There’s nothing “natural” about our current state, since it isn’t where we started out.
Of course there is still the question as to whether God would ever let someone fall completely out of existence.
As for the post-resurrection life, I am convinced that Christus Victor argues against all forms of annihilationism.