Currently using bitcoin for payments changes nothing. Yes, the transfer of "currency" are decentralized, but to cash it out (to pay for food, rent, etc) you still need to use exchanges which are centralized.
A) Regardless of whether you're right or wrong, that's just a discussion about labels. Call it whatever you want, my point still stands: the transfers are decentralized but the exchanges are centralized.
B) Even though my point does not depend on whether you're right or wrong, you're wrong. I have USD in my bank but I have to cash them out into GBP. So according to your "reasoning", USD is not a currency? Of course it is, you're just talking nonsense.
Either a currency is something you can buy things with, or it's a label with exchange rates. In the first case it can't be something that needs to be cashed out. In the second case you can't buy something and the thing you buy other things with is money.
You're usage of Bitcoin, money, GDP, USD and currency is willynilly.