I'm a .Net web dev who dabbles in WP development and I've been having just this sort of debate with myself about the direction of Microsoft's mobile APIs and technologies. First and foremost, I'm invested in Web tech. MS is quickly and steadily improving their ASP .Net tools and tech, so I'm not worried there. It's just that they have a long history of veering off in different directions and letting their old tech wilt while they hype the Next Big Thing and subsequently drop it.
Back in the olden days of Win32/WinForms development MS released WPF as its new way to develop Windows apps. It was big, impressive and innovative. The corporate world adopted it, but how many world-busting WPF apps are there? I can only think of Visual Studio itself. Then there was Silverlight, the cut-down version of WPF for the Web. It was adopted a bit in the corporate world, but another dud for non-corporate devs. As Silverlight's star was dimming, MS decided that Silverlight would be the basis for their new Windows Phone product. So far, all based on .Net or a version of it. By Silverlight 5's release, it was clear at the dev conferences that the interest in both WPF and Silverlight were close to zero within MS.
Then Windows 8 happened. A brand new WPF-like API that was not based on .Net whatsoever was released called WinRT (along with a confusing product confusingly named Windows RT; good job, MS). All aboard the new development train! Of course you can only make full-screen Windows 8-only apps with this new API. Furthermore, everything above is relegated to the "legacy" trashbin for desktop development. The strategy started to get a little clearer with the release of Windows Phone 8.1, the announcement that WinRT apps could be run on the Xbox One and a preview of the new Start menu and windowable WinRT apps in what would become Windows 10. Build 2014 actually felt kind of exciting to a MS app dev.
Now with the recent VS announcement of support for Cordova and Xamarin (don't get me wrong, I like this and I like bringing .Net to Mac and Linux) and the real sense of disinterest I'm getting from MS in Windows Phone and even Windows itself as a development target, I'm beginning to wonder if I should just move on now. MS itself has been concentrating on other platforms and their newest apps have been obviously designed with other mobile OS brands in mind (see the hamburger menus popping up in apps like OneDrive and Health). Is this an indication that teams within MS are using Xamarin internally and not their own native toolkit?
So where to from here? I'm doing app development for fun (the Web is my day job), so I don't have much invested in it. I can afford to keep plugging away at WinRT until MS figures out a direction and/or deprecates WinRT in favor of something else. For you, it seems that Xamarin is the best option. It keeps your options open in case MS decides that WP should go the way of Windows Mobile, Zune, Kin, ASMX, Win32, WCF, WPF, Silverlight, HttpWebRequest, WebClient, and Linq to SQL.