Nope! I don't think cash will be destroyed. I think if bitcoin will destroy cash in the future the government will act before it happens, in that way bitcoin will be destroyed first. Why? Because cash is what most people needs, those who don't have the bitcoin need cash. Those who are homeless needs cash. There are so many people who need cash rather than cryptos. How do you think they will live if there is no cash? You can think of other of other scenarios. Do you think bitcoin will not be banned in your country if they know that it will destroy the cash someday? I think it is better that they can coexist rather than destroying each other.