Banks are not going anywhere even if bitcoin gets on the mainstream there will be many people who will prefer to use banks and save their money in it as they feel that banks are the most safest place to save their money which is actually true.
This is true in more economically developed countries but there are many others where you can't rely on banks and in those countries Bitcoin is a better option. So Bitcoin will never make banks disappear if even in undeveloped countries situation will be changing for better, but if in countries like USA or Germany banks become unreliable then Bitcoin might replace them, at least to to an extent.