Cash isn't going to disappear. Not now. Not as electronic currencies get more advanced. Not when your mother starts using Bitcoin. Human beings will continue to use currency for a long, long damn time to come.
There are simply too many niches in an economy that using physical, anonymous objects to represent economic value fills for it to fade entirely.
I bring this up because every once in awhile someone suggests that cash is on the way out, or outright encourages it. With Bitcoins' recent price surge, even as governments look at ways to regulate those that use it, there's more talk about how close we are to a cyberpunk cashless society.
Yes, potentially, if enough large sectors of the economy agreed all at once, the transition could go a long way to squeezing cash out in favor of say, Bitcoin or one of its eventual descendants.
But realistically companies start accepting Bitcoins out of convenience, and will accept their regulation for the same reason: no one wants to be the sacrificial lamb to the government. If you don't believe me, ask Wikileaks who won after national governments started leaning on the companies it relied upon.
Even as anonymous online currencies become more advanced (and potentially more appealing to some sectors of the grey market), hard cash occupies too much of an easy route to fade entirely.
Put it this way: it takes serious, ongoing technological innovation to try and keep an electronic currency one step ahead of the state, while it take serious, ongoing resources from governments just to feebly track the existing under-the-table cash markets. They're probably not in too big a hurry to turn over every stone, either, as the black market helped bail out the legal one during the last financial crisis.
It's hard to beat the basic fact that you can hand a wad of faded paper to someone and they'll give you something in return. Hell, if you really want to get innovative, you can even use dish detergent.