I also have been working as a software dev for more than 10 years (and as a hobbyist game dev wannabe) I've had very little experience coding to a closed system, but with current games complexity I can't imagine it being all about going super low level code that won't be useable any where else.
On 360 we had devs (even first party) not going all in into the architecture because they knew it would be a useless effort. Then you have game engines providers who only need to do the hard work once.
And also, the uwp is not a solution for having the exact same code on all platforms, is a solution to share the most of the code base and deploy a single package, instead of going through many certifications processes. Specialized code for each platform is possible, but it enables to be done only when needed, instead of changing the entire code to a different API.
How well Ms will succeed remains to be seem, but in their favor, some games have been ported fairly quickly to the new app platform and the performance seems great on the Pc side, though there are some issues with Gears Ultimate in that regard. The big question is how well it runs on the Xbone side, they haven't showed much yet, even universal apps on xbone we kinda only get sneak peeks, but I doubt they would go that route if they weren't absolutely sure that performance wouldn't be a compromise, because they can't afford to concede an even greater advantage to sony in that regard.