They aren't abandoning it, they're just optimizing for the use case. In fact, Silverlight developers should be happy - with some simple namespace changes, they demonstrated converting a silverlight program to a Metro UI on stage in the keynote. Sure your program may not run in the browser unless they swap to full desktop mode, but it will be easy for you to make it a proper installed application if you desire.
It's a good decision in my book, and will result in a better tablet experience. Frankly all the misinformation and hyperbole around this is getting exhausting. They've really not pulled the rug from under anyone at all.