They will take the risk of upsetting desktop users to get the tablet and mobile 'phone users.
Will the enterprise user put up with it though.
We won't know for a while. Enterprises can take a long time between updates. Many are still on 2004/XP desktops, in the middle of migrating to Windows 7.
My crystal ball says that early adopters of Win 8 will scream blue murder and revert to 7 until 9 comes along. MS secretly know this perfectly well. But it is bad marketing to diss your shiny new product, so MS will not admit to the long 7 life span until after the screaming starts. That's 'cos this is just a carbon-copy replay of the Vista fiasco.
At this point my crystal ball swerves over to a rumour that the Surface will have a four-figure price tag, even the cut-down ARM version. So no expectations of pile-em-high-and-sell-em-cheap sales volumes there, then.
In other words, the Win 8 market appears dependent on OEM sales to the failing Nokia and the squeezed netbook/wannabee tablet vendors trapped in the MS ecosystem.
The ball sees Nokia crashing in flames, the netbook/tablets fighting a gallant rearguard action - in steady retreat unless and until Metro and MS Office-in-the-cloud can play cleanly and cheaply with legacy Office-in-my-network business systems.
Could Win 8 be just a marketing ploy - spinning the "win 9 aplha" as a product launch in order to put people off iPads and Galaxies, buying time for that to happen? I don't know, the crystal ball's battery has just died.