Just 4 months ago they announced they were suspending development of OneNote Win32 in favor of just the UWP version [1] Moreover, they have (had?) an entire multi-year roadmap planned around UWP, including the eventual release of a version of windows which was supposed to substantially reduce day-to-day reliance on Win32 for lower powered devices, codenamed Project Polaris [2], in an attempt to compete with Chromeboo…
It makes more sense to gradually rewrite portions of it but that doesn't work well with the annual release model. It does however work with the O365 continuous release model they adapted from Windows 10.
Just as they're gradually porting Control Panel and other legacy Win32 apps to the Settings App. I could see them gradually convert Office to modern cross platform frameworks.