It's not that they didn't have a plan, it's that the plan was so thoroughly ill-conceived and ill-executed.
iOS has not been unified with OS X, and Android has not been unified with Chrome OS. And all four OSes are quite successful despite the lack of unification. Meanwhile, the Windows RT Surface tablets never took off, and the Windows 8 unified experience was universally recognized as an unmitigated disaster on launch day, if not earlier.
They were always going to struggle to get app developers to port to Windows Mobile, so I'm not sure it was ill conceived to provide a single target for desktop and mobile, with the hope that more developers would take the time to support all form factors if they could use a single code base.
It was a gamble that enough native desktop apps would remain for this strategy to pay off, but they lost to web and electron apps.
iOS has not been unified with OS X, and Android has not been unified with Chrome OS. And all four OSes are quite successful despite the lack of unification. Meanwhile, the Windows RT Surface tablets never took off, and the Windows 8 unified experience was universally recognized as an unmitigated disaster on launch day, if not earlier.