When we can control all financial functions easily then we no longer need a central bank, the modern era that makes us transact internationally requires a currency that can be accepted globally and that of course cannot be controlled by the central bank, and this is what makes one day central banks die because crypto users are unstoppable.
Banks may be unnecessary for individual citizens. And then, it is unlikely that such citizens stop fully using cash that is issued by the central bank. No matter how much the number of cryptocurrency users increases, this will not affect the operation of the banking system of any country. On the contrary, banks are under the protection of the state, and it is capable of eliminating any threat of their existence by its methods of regulation.
So far, the G20 countries have concluded that cryptocurrency does not pose a threat to global financial stability.