The comparison isn't valid. I can compile "hello.c" and run it on multiple platforms. But will it work with existing production code? There are Silverlight specific libraries, for example.
And what do you mean with existing production code? I imagine you can run still run it in the browser like it's currently working.
I'm not a Silverlight developer, but I just can't understand the big commotion. Were people expecting for only SL to work wtih W8? I haven't heard a single thing in the keynote to even hint that SL is going away. Not only that, but they made a point to show that the current investment made in SL paid off. What exactly should they have done?
When a ship sinks, you save the women and the children first. As cruel as it seems, the reality is that people who have few years left are considered last.