See, I don't think there will be a successor. People will choose what they want to use, thats what a free market for money without government coercion looks like. People like you and me already live in that world. You choose bitcoin, I choose different things for different reasons, including bitcoin but mostly monero. If I wanted to buy something from you and you only took bitcoin, you'd get paid in it. We can still talk about the merits of this and that and why we each choose what we choose, but the point is we get to choose. That's what it means to live in a non fiat world: not using money by decree.