Bitcoin is the biggest bank rival at the moment, if Bitcoin becomes more popular and widely used for long-term asset investments, of course banks will experience financial difficulties. Currently, the Bank's income is almost entirely from customer saving, if one day the customer prioritizes saving money in the form of Bitcoin, of course the Bank will experience financial difficulties and will have a bad impact on the bank's financial condition in the future.
Until now there are still many countries that do not want Bitcoin as legal transaction, this can have bad impact on bank consistency if Bitcoin has become legal currency.
Banks and Bitcoin are in a competitive relationship, and Bitcoin is the biggest competitor of banks. Bitcoin is decentralized, there is no control over the assets, and now more and more people prefer cryptocurrencies and keep their money in their wallets instead of banks. Banks want to control the money, and the advent of Bitcoin took away the power of the banks over the people, and they gradually lost customers and revenue.