We don't need a new OS but I can understand the need to technicaly unify the coding platfroms through various processor architectures hence the need for WinRT on tower PCs. The need to have a same app, a same app version, working and being installed on the whole range of devices from the smallest to the largest.
What we DO NOT need is to change the way we interract with the computer because that's already at the optimum (Microsoft fails to understand that). There is nothing easier or faster than resizeable windows, jointly with a task bar and windows captions. Instant drag&drop, copy-paste, dialogs, moving files from one folder to another etc are of course irreplacable.
Forget about touch screen replacing keyboards and mice. A touch screen is cool when you are on the bus or at a diner and you can't deploy all these gears without being ridiculous, but in all other circumstances the keyboard and the mouse are far more faster, accurate and reliable. We don't need to handicap ourself just for the sake of looking modern.