Bitcoin has been proven to be able to make customers switch to crypto, this is what makes the central bank worried if crypto continues to increase, complete money control makes anyone not worry about saving their money into crypto and of course this will make the central bank collapse.
I think Central Bank is not yet worried about cryptos because crypto is not yet declared as legal tender or as a mode of payment (here in my country) and so no any kind of business is accepting crypto payment. Fiat money is still backed by the government and is still recognized as legal tender. Crypto has a long way to go before it can collapse the Central Bank.