They already did. From my memory, there is several major banks, including Central European Bank, which clearly said to people that it was dangerous to invest in Bitcoin, and that they should focus on their investment plans with ridiculously low yields.
yeah, some banks do such things to make their customers still on the point that they must keep their savings in a bank so that bank will profited. You know what everybody wants to make their business continue, so for this reason they talk wrong about the factors which effects their business like bitcoin now is big threat for banks that is why banks started war.