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.
The number of coins is definitely not a problem as the higher the price goes, the lower the amounts that people have and use will be. Add that we most likely won't even reach 1% (+70 million) of the world population to use Bitcoin. Right now we have just a few million people at most that are using Bitcoin on a regular basis. If we see this number go x10 to around 25 million users, then the price will go up to much much higher levels. Right now the majority of the people don't even have 0.5 BTC. Imagine how that will be after a few years when the price breaks through ATHs constantly.