It'll never be mainstream. Whats mainstream is the current clearing houses that banks use daily.
Cryptocurrency is a tool. Hell, I bet even some entire countries are moving funds around the world globally with bitcoins on the down low specifically to avoid banks. But it will never replace banks.
One reason is the same reason for bitcoins scarcity: you lose it and its gone. Screwups happen and banks are able to adjust their ledgers to make good on mistakes and fraud. Money out of thin air as people complain. Bitcoin will never be like that. It is entirely at your own risk.
[shit... my dog hit my keyboard and the quote got all screwed up and I lost the intro. I can't even remember what I wrote, so this may not flow as well, oh well]
BTC set out to be a currency, but I think if people want it to stick to these high levels, the currency aspect needs to be dropped.
But then there are the cryptos that are better described as utility tokens. They can be traded in retail but they are actually more specifically made for concepts like smart contracts, and platforms like Ethereum and NEM could easily have a Ripple-like token on them that banks use to move currency around.
I mean sure there are clearing houses, but they are relatively slow in the grand scheme of things. Those same transactions can be initiated in cash, and the banks basically use a distributed ledger and it's utility tokens to actually issue the transfer instructions immediately and verified by all participants. It could effectually replace the entire clearing house system, at least in some respects. Clearing houses and central banks will still play a role but even further up the chain, all the bank to bank stuff can skip it entirely until regular settlement periods, but due to the immediate verification system, banks don't have to "hold" the digital representation of funds until settlement. They'll all eventually settle regularly with the central banks but the banks don't have to make the customers wait for settlement.
Hell, that could actually make the regular asset markets faster too!