Will never happen. People, collectively, will "waste" computing resources regardless. For example: http://www.distributed.net/RC5. Yes, I spent countless CPU years on that as did thousands of other people. All crypto currency is is just an extension of that which makes it possible, although not always probable, to break even on the cost of the hardware. See also SETI@home, Prime95, Folding@home, and dozens of other similar projects. You can't ignore the addiction factor, and no amount of convincing people of negative ROI will help.