I'm not convinced that OS'es are becomming less and less relevant as simplistic cell phone OS'es are becomming popular as more people buy a smartphone.
Windows 8 took this approach in order to create an OS that looked and felt like a tablet OS on a desktop computer (and thus would attract people who wanted to use one OS for everything) and it failed so badly that they had to revise that policy. People simply hated the idea. They do want a traditional OS on a computer, just as they want a cell phone OS on a cell phone.
Traditional OS'es are not becomming irrelevant, they are simply adapting to new technology.
