the current distributed nature of all crypto-currency including bitcoin is probably a threat on how banking system operates. current banking system usually is a centralized system, where all trust is placed on the banking institution. for this reason itself, crypto currency model will not be adpoted and accepted by banks, however, its in their best interest to regulate it though.

i think, in the future, in some way, the currency will shift towards being digital currency. in any case, most of the transaction we do are virtual any ways, i.e. using cards.

The very nature of crypt-currencies reflects that we should not seek for a bank/government-level regulation so, I think the answer is yes; the crypt-currencies will be widely used but waiting for banks to support it? I think it's kind of against the philosophy of this new currency.

I think it can not be accepted unless it is regulated by any trusted body. Otherwise it may affect the economy of any country as well common people may be bankrupted. It's my assumption nothing else.

It's very difficult to pinpoint what could lead to the adoption of the Cryptos. It's a completely unfettered market and that's exactly what makes it interesting and different than the regular currency. So, to begin with, if more merchants are willing to accept the transactions in Cryptos, sure! It'd be all over, in no time. Also, the Govt. has to give a go-ahead. But it's quite unlikely that the regular currency would be replaced anytime soon. I'd resonate with the answer by Rupak above, that it'd definitely shift towards Digital currency but adoption of Cryptos, doesn't seem to be happening anytime soon. They're too volatile to be accepted as a stable currency!

The widespread adoption of cryptocurrencies by citizens pose a threat for banks and governments. It will be hard to introduce legislation / regulation and keep the original spirit of a decentralized peer-to-peer model.