It's been said. But according to some of the hardest science fiction, we should have flying cars, talking computers, and people living on the moon by now. Stanley Kubrick and Arthur C. Clarke made 2001: A Space Odyssey, the most realistic SF movie to date, in 1968--but they couldn't have predicted a number of major events, including the microcomputer revolution and the end of the Soviet Union, which turned world history in a completely different direction. (To say nothing of minor events like the breakup of the telephone company or PanAm going defunct.)
The problem with "AI will take over the world," as I see it, is that the human brain is so much more complex than a computer. If you networked all the computers in the world together so they could function as one, you'd probably have the equivalent of ... a couple of people. And they're probably still not capable of thinking abstractly. Some SkyNet.