The flawed assumption is based on the myth is that all operating systems are the same. They aren't.
Software design is the top factor for vulnerability - are Macs are designed to be more secure than Windows. If everyone dumped Windows and switched to Macs, the number of viruses, worms and spyware in the wild would plummet. There would be a huge leap in human productivity by recovering billions of man-hours that are currently being wasted on Windows malware. Billions of dollars wasted on troubleshooting unnecessary Windows problems would be saved.
As for Linux, there are many different distributions available. Out of the box, some of them are probably more secure than Mac, some less. But the Linux user interface is awful for the average computer user, and configuration and maintenance are difficult, so there is no improvement in productivity for the average computer user to switch to Linux. Linux is best suited for servers and hobbyists, where the users are willing to invest the extra time required to learn how to manage it properly.