Actually, a lot of what you see in Windows 10 is running on top of UWP.
Non-sandboxed applications and services are not a sustainable strategy anymore.
WPF was XAML v1. Silverlight was XAML v2. UWP has XAML v3. Your skills are still applicable.
You can also now use it with both native C++ and C# thanks to WinRT.
The Office version you get with Office 365 ("Desktop Office"), the one, businesses use, is certainly not UWP.
Skype UWP is a preview announced a few days ago.
"Are all using UWP"? No. They are in a transition of moving to UWP, but every serious business application is Win32.
It's a hybrid: start menu, action centre and some other elements are UWP's XAML frames now.