I'm sure the big private investors will already have their share of Bitcoin.

The retail banks are really middlemen, rather than investors. Their risk is that you no longer need to use their bank facilities or currency exchange or the savings plans and mutual funds that they offer and on which they take their spread. Also that they cannot make a margin by lending out Bitcoin that people have invested with them. I think it will be a lean spell for lots of the big banks but then they will adapt. There are plenty of services they can offer in the decentralised world.