If banks join crypto world, we will have a huge problem, because banks like power and control, soo they will manipulate crypto. I hope they stay away since bitcoin isnt really a big problem to them, since they keep with their services of savings and lendings, something that at bitcoin is just possible the savings, because lendings is a high risk attempt, and even with some people lending here is for reputable members or with proper colateral.
I don't know how can the banks control bitcoins when it's unregulated and decentralized. Either they stay away or not, bitcoin will be safe from them as they cannot control it. They cannot blackmail or bribe someone since no one's in charge. They cannot pay the founder of bitcoin because there isn't any.
Bitcoin isn't a problem to them but when the number of bitcoin users blows up, it will start becoming their problem. They cannot tax the sales going on in the bitcoin world and when people start doing business online instead of offline (fiat), it will be a problem to them since they will collect lesser and lesser tax.