Lose them to who? Is Linux or a Mac really a viable upgrade path for most corporations? How much Windows expertise has been built up over the years/decades? How many interoperating windows products are they already running? Legacy software? All of these are major barriers to switching.

And this is the key to Microsoft's strategy with Windows 8. Roll out their next generation paradigm while piggy-backing on the momentum of their current generation software and infrastructure. If they had waited too long to transition, it would have been a situation of "well if I have to relearn everything anyways...". Forcing this paradigm on their desktop and server customers now was key to keeping the majority of their userbase in the transition.