Granted (literally IRL): 50%+ of Internet users are on mobile devices, 70%+ of those are running Android, and Android uses the Linux kernel, so Linux market share is already over 20%.
It's just that it's DRM'd and un-Free bastard Linux, not GNU.
Alternatively:
Granted: Android marketshare drops from 70% of the mobile market to <40% (so that the Linux marketshare across mobile+desktop no longer exceeds 20%), and fully-proprietary OSs take up the slack.