UWP is the right thing to do for MS, they have one of the most used and complete API, direct X, a stronghold on PC, they now have a common platform, so definitely that can improve their shares in gaming, or create new opportunities.
But I don't think it can have a tremendous, immediate impact. they are currently not growing on their strongest markets (console and OS), challenger on others (smartphone, tablets, but also universal API, and Store) and whatever the universality you provide for your binaries, API, and store, at the end of the day you have totally different constraints on each device : length of gaming session, controller, screen, power, battery if any.
It's not by accident there is very few games that have been successful on that different devices. In fact I can think of only one, Minecraft, perhaps that's why spent so much on it.
