Replaced? You mean the imacs?
Not at all. I am referring to the marginalization of the desktop computer of any type (including true laptops) in favor of computer appliances, such as smart phones, tablets, consoles, and other devices. This isn't happening as a top-down sort of change. Rather it is consumer-driven. We have already reached the point in the U.S. where more people have such devices than have computers and the majority of people in the U.S. now access the Internet exclusively using smart devices rather than computers. (I'm a former journalist and a webmaster, so I've kept up with these statistics.) Unless the trends change, I suspect computers will again become a niche product for the few who really want one. The rest won't care.
Meanwhile, the operating systems of Windows, OS X, and Linux are moving more and more towards a smart device-like interface rather than a more sophisticated and complex traditional one. Whether they're responding, leading, or a combination of both, the direction is the same.