UWP is the future, Win32's long term roadmap is to join Win16.
In any case, one of the points of April 2018 release and the upcoming Redstone 5 one is that those models are being merged.
And yes, the next Office version for Windows 10 is store only and as usual in Office tradition, is the one responsible for many new UI controls (UWP Fluent Design) in the upcoming Redstone 5 version.
