We're moving into an age of five different types of inputs:

    1. Keyboard
    2. Mouse
    3. Touchscreen (finger, stylist)
    4. Audio (e.g. Siri)
    5. Video (e.g. Kinect, eye tracking)

In video games, going all the way back to the Atari 2600, we've seen plenty of new types of input controls tried and failed. Joysticks, rotary controls, gamepads, keyboards, mice, trackballs, touchpads, light guns, robots, power gloves, giant floor-mats with buttons, analog sticks, analog buttons, vibrating controllers, skateboards, drum kits, guitars, microphones, steering wheels, arcade sticks, etc...

Some were great, some were terrible. The "great" tended to be so because they were tailored to the software they were programmed for. Playing Guitar Hero with a guitar controller is good, but playing Call of Duty with it is bad. There are plenty of different types of inputs that exist and will be invented, but some work best for some things and terrible for others.

As you mention, Microsoft Office is designed around a keyboard and mouse. A touchscreen is a hybrid of the two, more portable, but clunkier. An accountant who lives by arrow keys, hot keys, and their numberpad in Excel is going to hate a touchscreen. For MS Office to work on a tablet, it'll have to be re-designed from the ground up, and even then it may not be superior to its desktop counterpart in an office setting.

I don't know what MS wants to do with Win 8. To think all desktops are going to die and become tablets is wrong, just like thinking that T.V. was going to kill radio. Both have their places, but a tablet != desktop.

Maybe you're right that they just want a "quick success", but that strategy hasn't paid off for them recently (Zune vs. iPod, Bing vs. Google, their fragmented mobile efforts vs. iPhone). MS has been in reaction mode for a while, and now they're reacting to the iPad with Metro.

One thing Microsoft does have going for them is the corporate market; Apple hasn't directly targeted that yet, though to think that Apple is not going to go after it shortly is foolish. MS is entrenched in here with Active Directory and Exchange. But cloud efforts are going to shortly give that a run for its money.