Meh, I think it will be a long time before that happens. The thing nobody likes to talk about with digital currencies is... well, to put it simply, no one is spending them as currency for the moment. Everyone is hoarding them. They're not being used as an exchange medium very much.
Now with that said, they are definitely here to stay, and they're going to be one of (key words: one of) the things any investor needs to add to their bag of assets if they want to stay competitive, just like stocks, bonds, and commodity assets.
Ask this question about digital currency replacing standard currency about fifty years in the future and it may result in a very different conversation, but for now... well,
"hodl on just a minute here."