The tablet thing is significant but I think a lot of people come at it the wrong way.
It's not that tablets and smartphones are replacing computers for people who want computers.
They are replacing computers for people that want to do computer things, but never wanted a computer otherwise. Facebook, email, instant messaging, basic web browsing etc.
There's a wealth of people I know that barely use their computer except to write the occasional letter, but they are online almost every waking hour on some portable device.
When you factor in that in that environment, manufacturer/vendor lockdown is widely accepted, and piracy is a lot harder, it becomes a nobrainer for someone like microsoft to follow the trail.