Given that there are already enterprise users out there today on Android and iOS devices, this is a pretty bold claim. (And you can, in fact, plug things into tablets with both operating systems - they have USB support.)
These are not all being deployed as "secondary devices" - in many cases companies deploy them because they're cheaper to deploy and maintain than a laptop, which only makes sense if you're not also providing a laptop to the user.
Having companies run solely on laptops or tablets has nothing to do with whether or not there are enterprise tablet deployments. Your logic is flawed.
Net impact on Microsoft's bottom line: zero.
google now says you interact with the tablet by voice, and you watch the screen with your eyes. Now, tablets can even derive events from looking at your face (samsung you look away video stops).
Noticed you don't need a mouse with a tablet?
Oh, the software will have to be rewritten, no doubt, but since most of the software is interface anyway...
really? What can you do on your tablet in enterprises? You can have them only as an additional device. They don't even have any port where you can plugin something. And, you will have your tablets interact with whom?
At the end of the day, you need real machine running full blown OS to get shit done and not devices meant for playing with those face painting apps.