Isn't it partially the opposite though? Properly written UWP apps are supposedly super-easy to adapt to run on mobile, so really the promise of WoA is flirting with the idea of not having to even convert to UWP. Unless I'm missing something here.
Centennial apps and win32 don't easily adapt. There's a whole porting process that Kodi, is for example going through. First they centennial bridge, then they port to UWP proper.
Check out the article on the mainpage on Kodi, to see a little of the process.
It's made simpler by MS, but its not just a matter of clicking a few buttons, you need to convert your APIs, device and port calls etc.
The promise of WoA, as far as I can tell is the chipset. By putting arm chips into tablets, notebooks, and hybrids, windows devices gain all their hardware benefits - slimmer forms, longer battery life, cheaper price points, built in LTE/telephony and GPS.
In this way, it serves as windows s does to the centennial bridge, to the islandwood bridge (ios ports). Developers can port their mobility focused ios software via the islandwood bridge, and then code for all three platforms via xamarin 2. Cheap tablets, hybrids and laptops all with always connected LTE, calling/SMS and GPS is a motive for ios developers to create full UWP apps, the same way windows s is a motive for win32 developers to at least come halfway via islandwood.
UWP is the future. It's just a while to get there. WoA serves as another bridge for legacy software, whilst encouraging non-win32 developers on ios to get their arses over to the windows platform.
MSFT doesn't want everyone to stay on win32. It's just a big thing to adapt to, so they are creating all these bridges, software and hardware.
WoA tablets in themselves are a 'bridge' to smaller devices. You'll note windows proper is slowly scaling smaller. First surface. Next will be surface mini or similar.
All going well the windows store should look much healthier next year, especially late next year, including for existing windows 10 mobile users.