I don't think it is so now, but in some distant future this might become true.
Broadly, banks make money by issuing loans and placing deposits from customers as well as providing some settlement services, etc. and they charge commissions on all their operations. The commissions vary by country, but they're significantly higher than the cost of a transaction in BTC. Of course, banks will start losing money they used to have from their customers because in order to stay competitive they will have to lower the commission amount, etc.
So, BTC is not a disaster for the banks now, but it might turn out to be a disaster in the long run, I suppose.