Bitcoin will never make banks disappear. That is impossible. There are only 21 million of bitcoins in the future. That is the maximum amount possible if i am correctly informeted. So if everyone in the world should be paying with bitcoins, we should all have very very very small amounts of bitcoin. That is impossible. I Believe in bitcoin as another value with which you will be able to pay all over the world, as you do with doller, yen, pound, euro or so. All the values will have there own life as now, and nothing is going to disappear.
While I agree that bitcoin is not going to make banks disappear I don't see how the 21 million limit of coins is any problem, does that means the users are going to have very small number of coins? Yes, but bitcoin can be fractioned very easily, after all, it was not long ago someone bought a pizza with tens of thousands of bitcoins.
21 million btc can be broken down into mbtc or satoshi. And if there is a need, it can even be further broken down to meet the rising demand.