It depends - are you willing to lose a significant portion of your potential customer base by using a newer technology (UWP requires Windows 10 - less than 15% of the internet marketshare for PCs)?
Microsoft also tends to create abandon rarely used technology after a few years. If UWP doesn't catch on, in 5 years it might be worthless knowledge. But they can't really abandon Win32 - it's core to their platform.
15% of share (a lot more if we speaking about gamers) is not bad at all if we consider that Windows 10 was released less than a year ago.
When Microsoft decided to abandon a technolgy, usually it continues to work without any issue on future versions of Windows. Keep in mind that UWP on C++\CX and WinRT, are actually all based on COM and Win32. This mean you can target UWP with "pure" C++ if you want, however this need actually a lot of work and you can also use the WRL template library to mitigate that.
On Windows platform Win32 is like the CRT: they are both the meaning of life for the Windows platform itself, you cannot kill Win32 or the CRT without killing the entire Windows platform.
A guy called Kenny Kerr started writing a pure C++ library called "Modern" to target WinRT without C++\CX and to replace WRL. It also joined later Microsoft, so hopefully Microsoft will release in the future a complete version of "Modern" to target WINRT and create UWP application using pure C++ without the proprietary component extension (C++\CX) or the usage of WRL. A library like that would solve all kind of issues regarding targeting both UWP and Win32 desktop application.
15% - you're right, it's not bad, it was just a point. And it's likely to go up quite quickly. I hear it's a pretty solid OS, but I'm sticking with Win7 for now. You're also right about marketshare among gamers probably being higher - but that's only because it's the version of Windows that's already installed on their gaming rig (I imagine DirectX 12 might be another reason. Was there some technical reason for Microsoft not backporting DX12 to Windows 8 and 7, or is this just an "upgrade already" thing?)
As for Microsoft abandoned technology continuing to function properly on new versions of the OS - try playing an old game using DirectDraw on Windows 8 or 10. Heck, they messed with DWM in Vista and 7.