Central banks are now the dominant structure that nations use to manage their economies. They have monopoly power and will not give up that power. While the adoption rates for Bitcoin and other digital currencies are small and the government's support for them is virtually nonexistent. So it is impossible for BTC to kill banks, which in my opinion is most likely one day that central banks will issue cryptocurrencies.
You cannot completely take that out, central bank might one day give up to that after they have been pushed to wall, because cryptocurrency is what they cannot cancel, and since they cannot cancel it, it will gradually spread and go beyond their control, you might see the adoption as little now which is nothing compared to what the adoption rate of bitcoin would be in the future.
If you doubt this, why do you think that Zimbabwe came out with their own cryptocurrency, because when you look at central bank, you are already looking at government, and for them to have released their Petro coin, they must have all discussed about it, look at china also coming up with their own DCEP coin too, china who happens to be the largest country, so talking of central bank not supporting cryptocurrency in future is what I doubt.