If they miss an inflection point, they could be in deep trouble.
At that point their sales are declining as people move off desktops, and they face the much higher bar of having to convince people not only to buy computers with their new OS but drop their investment in other platforms (such as apps purchased).
At some point they won't be able to count on their market still being there by the time they have an OS people want.
So to clarify, my opinion is that they're probably pretty safe as far as maintaining their position in the PC world even if Windows 8 bombs as a PC OS. Losing entrance into the tablet world for two extra years on the other hand is a lot worse since it would be very difficult to get those two years worth of tablet buyers to switch to their platform after having already bought into another during that time.
How the second part actually plays out in the real world is really hard to predict since a lot of it will be based on the hardware that runs the OS. For example, I personally think Windows 8 is a pretty poor Desktop OS, but I'd definitely consider it when buying a tablet. So my problem isn't so much the OS but their current flagship device; the Surface RT looks to have too many deal breakers for me and I don't think I have a need for the Pro version.
