if we continue on the path of enchuching onto the hegemony of USD they will shut it down. All they have to do is to say it's illegal and it would be game over for bitcoin.
Although it doesn't fit within what was traditionally defined as a currency, as in a physical object/note/coin, I think we all know that in some ways it is. It's just innovative and the powers that be are therefore having trouble defining it. As you said, if it can be exchanged for something, it's currency. But it avoids the definition of currency by not exactly existing as anything really. That's the beauty of it.
Coinbase quote on Bitcoins possible VAT exemption because it's not a currency:
http://www.coindesk.com/european-court-of-justice-official-proposes-bitcoin-vat-exemption/'Bitcoin operations should be exempt from Value Added Tax (VAT), the Advocate General of the European Court of Justice said in an opinion document published today.
Citing existing exemptions for currency and money transactions in Europe’s VAT Directive, Advocate General Juliane Kokott urged the court to opt against applying a tax to bitcoin purchases and sales. She further argued that bitcoin,
while not legal tender, is still a form of money.'
Even they are confused... 'not legal tender, but is a form of money'
I think quite possibly this legal ambiguity was deliberate in the design by Nakamoto for just this kind of reason.
As to the US, their constitution allows them to mint their own coin. It's up there with the right to bear arms, only not as well known for some reason. The US government would have a nightmare actually trying to ban it I would imagine.