Silverlight was available on Mac and Windows, and Moonlight was available on Linux. Again, the plugin itself didn't have to be open-sourced, the format was, which is why Moonlight worked.
In my opinion having Linux support done by community and lagging behind in features and in release scheduling is not cross-platform, Flash had the advantage in this point, Adobe AIR also worked on linux until a point.
Do you think that having this platforms opened source would help or not the adoption? Looking at Java I see it is still going strong with plenty on languages, tools and libraries/frameworks still going strong.
My entire point wasn't that Silverlight was/wasn't open sourced... it was that the underlying structure was FAR closer to what I thought was a good package structure, compared to .SWF
