It's fairly simple in my eyes: banks are not necessary in a world where Bitcoin is adopted by the masses. Everyone would be responsible for their money, and banks won't be able to charge for services they should be providing for free anyway. I'm sure they will find a way to survive, but they would be nowhere near as powerful or profitable.
Bitcoin is itself a bank it takes investment and one can use at their own as mode of payment there is also no need of banks like that. Who needs banks to put their money when they have an golden opportunity to get high profit and as well as more eases as compare to banks, bitcoin is versatile and it blesses everyone thats why banks are having some serious concerns to it.