Android tablets, the iPad, Chromebooks and the new iPad Pro represent a significant challenge to PC sales and Microsoft's embracing of these platforms represents a strategically sound defense against what could prove to be death by 'good enough'.
None of the devices I've listed above are particularly brilliant on their own, currently you need to pair them with a 'real' computer to cover the whole range of functions that form the current computing landscape.
For consumers this may not be the case for much longer. There comes a point where these cut down machines become good enough that we no longer worry about the missing functionality.
For old Microsoft this would prove to be an enormous problems. By focusing on delivering Microsoft services everywhere it becomes less so. The partnership with Cyanogen, the acquisition of Android and iOS apps, appearing on stage to talk up the capability of the iPad Pro, all of this demonstrates that Microsoft now has a desire to chase software subscriptions and deploy it services wherever it finds a willing recipient.
Currently the best way to experience Microsoft's services is on a Windows PC, that may not always be the case. By offering Office as a service Microsoft has a clear view of how its subscribers access the service and where it will gain the most return from development dollars. If that proves to be iOS or Android then Windows users may find that they drop back in the line for updates.
Good enough might just prove to be the thing that kills the PC, rather than any specific replacement product. Microsoft has positioned itself to flourish whichever way that goes.