Yahoo was destroyed by Google. Myspace was destroyed by Facebook. While I don't know what a currency that could destroy Bitcoin would look like, there is a possibility that something new could be so innovative that it all but destroys Bitcoin. Even then while Bitcoin could be dethroned, I don't think Bitcoin would be destroyed. Bitcoin has inertia. It is going to keep going. Even if something else gets ten times bigger than Bitcoin, I think Bitcoin will continue to grow.