This is a stupid fear mongering to a non-issue.
Firstly, DX12 works in Win 10. You do not need a UWP app to use DX12.
Window's Store is just a distribution platform. It's got its ecosystem, just like Valve has with Steam overlays, trading cards, social, etc.
Nobody is forced to ONLY release games for DX12 to the Window Store for it to work. You can build it DX12, release it anywhere you want, in .exe format.
Microsoft's games like Gears or Quantum, they can release it where they want, because they own it. If they want it exclusive, then so be it. Consoles have had exclusivity for decades.
If you don't like it, don't buy it and don't play it.
And if EPIC has issues with it, I hope to see them push Vulkan and put their words into actions.