Well, if you discount the notion of God as magic, then the answer is God ends with the thermal death of the universe, and was born at its creation. If you accept brane theory and a four dimensional universe or something like cosmic foam and a multiverse, then God is constantly being created and destroyed along the decision tree. God in this thought experiment becomes a proxy for space-time.
If, on the other hand, you place belief in a being which exists outside of reality as commonly understood, then the question is meaningless. God, so defined, cannot be created or destroyed, but merely is. What its purpose is also becomes a meaningless construct, because that which is outside of causation must necessarily either be purposeless or the sole purpose possible.
A more interesting question to me is if God is not magic, what possible purpose can a supreme being fill. Yes there are gaps in our knowledge of how the universe functions, but a God of the gaps isn't very satisfying. A God whose purpose keeps shrinking isn't a God. So what would a God, living or dead do? If nothing, then they are irrelevant even if they exist. It just isn't plausible to me that the irrelevant is all powerful.
That kind of leaves God as something outside of reality. What would that look like? Myth I suppose. I don't discount myth as mere fable. The myths of people provide meaning, and direction to lives. Terry Pratchett supposes that God's are real as long as there are believers and when belief dies so do the Gods. When a myth loses its relevance to a society, it rarely dies but morphs into something newly relevant and co-ops the old into something the current society can use. But use for what? To create systems of ethics? To control the conversation? To lubricate the functioning of societal norms?
I think it does all those things which explains why myths and religions persist over time. So if God and myth were to die, would society be poorer? If God was dead would we reinvent something equivalent? I think we would, I don't believe that God exists in objective reality, but the idea of God, is alive and well in psychological reality. I am much more concerned that God is the psychic manifestation of power and is, as it has always been, a tool of manipulation by those who control the conversation.
Peace,
Julie