Ten years ago, were you saying the same about laptops?
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.