It is a bit more complicated than that.

As a very very broad simplification, the value of a currency is dependent on the economic output in the area the currency is used. If the output increases, but the available currency remains the same, there is a deflation and if the output stagnates or lowers, while the currency amount increases, there is an inflation. Both are very bad for the economy at high rates and the go-to-solution is to have a stable small inflation.

The current concept is that the banks create (book) money by giving out loans and covering those loans at the central bank, which in turn increases or decreases the amount of currency in rotation, by setting an interest rate for that. That system would need to be remodelled entirely, if there is a digital currency, that replaces the book money of the banks and would be similiar to cash in terms of simply existing without interest rates in either direction. Now introducing a digital currency with default and unnegotiable interest rates for currency control, will not be very favorable with customers. In the end the banks do the same, but the customers still have the feeling that they could just cash it all out, or go to a different bank with different conditions. With the design of the digital currency that could no longer be viable.

The issue isn't banks fundamentally. It is the way that banks are deregulated, privatized and run by greedy fucks, who know that there bank is "too big to fail", so they can get away with holding the real economy hostage.