He's completely wrong about that though. Show me people doing serious computing on iOS or Android. iOS is still so limited that despite having amazing processors in them, new 1000+ dollar iPads are essentially useless to anyone other than artists.
No one is talking about serious computing. It's about personal computing. As I grew up, kids, the lucky ones at least, had PCs, regardless of serious computing or just for IRC and porn. Nowadays kids are on smartphones, many of them almost exclusively. My SO doesn't have a computer anymore, my dad, his wife, my sister don't have one either. They have their phones (and tablets) where they manage their whole digital lives, photos, social networking, emailing, web browsing, video, the lot that apps and browser allows them to do. Curiously enough, my younger brother does have a PC, for gaming (even that, many kids just do on phones, tablets and consoles). Older generations that didn't "get" the PC now have a smartphone. Younger generations are growing up without any advanced concept of what a PC/Mac can do, besides basic utilization of a keyboard and mouse. The odds are stacked against the PC/Mac when it comes to being a primary computing device for the majority of the population and it will only get worse.