I was specifically using "successful" to mean "number of users." People use what they like, and Ubuntu has gotten a lot more users than, say, Gentoo. Is Ubuntu superior to Gentoo? That's a matter of opinion; I'm sure that there are plenty of applications where people have decided that Gentoo is the perfect fit for what they want to accomplish. But for personal computing needs, people tend to use what is most user-friendly.
I don't mean that it's okay to be completely illiterate in how a computer works, especially if you're using Linux. To take your car example, I don't think that a driver should know how to rebuild his engine. I do expect the driver to know how to change his oil and brake pads and know about scheduled maintenance.
The same is true for computers. A Linux user should know basic bash commands, how to install software, how to research problems, and so on. But he shouldn't be expected to know how to troubleshoot driver problems or know the ins and outs of xorg.conf. That's what Google and the aforementioned benevolent experts writing guides are for.