No, banks are already thinking into some way to use bitcoin technology to save them a lot money over their inside transactions. Bitcoin isnt a real problem for banks, they provide a usefull thing the loans for over 2 years up to 50 years, the max period you will lend your coins will be for maybe 2 weeks with colateral. Besides those the world economy does use banks, and they wont change it at all, soo bitcoin isnt a problem to banks at all, even with some people using it, the money will return to banks in the end.
What you're saying is true because people will also exchange btc to fiat in order to buy things that can only be bought by means of fiat money. But what everyone saying here is what if many countries began accepting btc as a payment ( as we all see they began regulating btc ), and what if many people are using btc let's say 60% btc 40% fiat currency, I wonder if the banks will accept the fact that btc is the new age and they will bought a lot of btc in order to replace their money into digital currency which I think if they don't do some companies will begin launching other cryptocurrencies banks and the older banks will fade, which I think this will have a lot of contradictions regarding fiat to cryptocurrencies to Central banks and to online banks and to government and it's people who use cryptocurrencies. This will be a long haul of hearings and New laws will be implemented in the payment system of cryptocurrencies.Your assumptions are valid but it is far from happening anytime soon because our financial industry, which banks are in, are regulated by the government and they can control and implement policies for the thriving of these banks.
