The reality is that powerful complex devices like a multipurpose computer don't work very well. It's hard to make good UI's that are right for all those different functions. It's hard to make absolutely sure that they work right all the time and never crash, because they're so complex. And it's hard to teach your grandma how to use them.
The exact opposite of that old prediction now seems to be the trend of the future - rather than 1 powerful computer, you will still have tons of single-function devices - they will just all be computers! A computer to watch TV, a computer for video games, a computer to browse the web, a computer for business software. They'll be very powerful, but very cheap, with special single-function software that's easy to use and reliable. This is of course already happening.