"therefore the only one that developers will use is Win32."
Moreover, most developers of complex GUIs will have their own commercial, free/open source or custom made Win32 controls for many different needs. Asking them to replace them just to use a new UI you may not feel compelling at all for several reasons, is akin to commit suicide.
It could be easy to convert simple applications with a simple GUI - but MS itself has dumbed down many applications in the process - Windows 10 Mail application is almost unusable but for very, very simple needs.
But as soon as an application need a more complex GUI, the new approach looks overly complex, with no benefits. Most commercial application don't need materials, animations, and fancy effects - they just need to allow the user to complete her tasks quickly with no errors.
Adobe may repackage Photoshop Elements and Adobe Reader for the Store, but everything else is going to require Adobe CC, and Adobe also has to care about having a similar UI on both Windows and Mac - I think it's very little interested in a Windows-only UI.