Back when the app was started, Silverlight had a good future, everything looked rosy, I could easily port my .Net skills to SL, and so that's the platform we chose.
Then came Silverlight 5, and all of a sudden the platform independence was gone, along with some reliability (MultiScaleImage crashes when I use SL5, in SL4 it's just fine), so as it is, my app is stuck at SL4.
Kinda between a rock and a hard place, because there is just too much time invested in this app to go for a re-write in Flash for instance.
Awesome move, MS.
On the plus, I have found a Silverlight project that emulates the HTML5 Canvas, so I might be able to run WebGL on that.... will investigate and feed back.