Why we mix these two things because central bank and cryptocurrency have their nature to serve. We need both of them now. Central bank operates the whole country money systems and controls inflation or deflation etc. Bitcoin never liable for central bank collapse while it may be an element of loss demand.
Yeah, this is another anti-bitcoin speaking point. Do not look at people who say things like "bitcoin is so awesome, it will ruin banks!!" they are not trying to say bitcoin is good, they are trying to secretly say "if bitcoin goes any bigger, banks will be gone and all of our money with it and it will be chaos" but they can't say that exactly so they hide behind things looking good for bitcoin and bad for everyone else.
Bitcoin will not ruin banks, there is banks that has 100+ different currencies in fiat, why wouldn't they add another currency called bitcoin? They will do that as soon as governments let banks become crypto exchanges basically. If one bank can hold both dollars and euroes that means they can hold bitcoin as well, all banks can do that. Which is why bitcoin is not a danger to banks, sure it could be for the small ones who ignore bitcoin, but all the ones that accept the fact bitcoin is too big to ignore will be fighting for it.