Cryptocurrency is not inherently a debt instrument and cannot simply be "printed" or issued past its limit. I know a lot of people here feel that banks should die and banks are evil, but realistically I don't see how an economy can survive without debt instruments and issuers. If Bitcoin does replace all currency, does this mean that banks will collapse, or issue their own cryptocurrency to 'lend coins' to people who would like to make a big purchase and pay it off over time? Would it even be possible to pay interest back on a large scale if a coin has a finite supply? If there is 21 million Bitcoin in debt lent out and circulating, how could there possibly be a way to collect more than that by the bank. Would multiple cryptocurrencies solve a problem like that, or would it mean that the debt just gets pushed off onto some other coin and then also onto another infinitely?
There is no reason why bitcoins cannot be loaned. I have loaned bitcoins to people. It doesn't seem impossible to me.
The argument that "if all 21 million bitcoins were loaned, it would not be possible to pay interest" doesn't make sense because the loaned bitcoins are not consumed or locked down and loans and interest are repaid over time.
Also, some people believe that it is impossible to have fractional reserve banking with Bitcoin, but I don't see why not. I fully expect to see FRB with bitcoins in the future.