Sorry for chiming in here without reading through all of the posts. I'm just going to throw in my two cents about why Silverlight needs to be made open source.
Firstly, I've been using Microsoft development technology since Quick Basic. My first job was programming in VB 6. I was converted to VB .Net, and then C#. At that time Windows Forms was the graphics library to be used. This was a step up from VB 6. Then, I detoured and dabbled in ASP, and a little bit of WPF when it became a thing.
When Silverlight became popular, it honestly looked like it was going to become the answer to everyone's development dreams! Finally, a powerful graphics library was at our disposal, C# code could be shared across platforms (including MacOS!), it could be deployed in a browser, and Silverlight generally became and amazing platform to develop on. I can safely say, it was the most mature UI platform I had ever developed on.
Silverlight lacked touch friendliness out of the box, but most of our users were touch screen tablet users. So, we got to work on making Silverlight touch screen friendly for our users. We developed pop out keyboards etc. that made Silverlight completely user friendly without a mouse and keyboard. What surprised me was not the lack of touch screen usability in Silverlight, but the ease with which touch screen functionality can be added to Silverlight.
Our software is still going strong on Silverlight, and although the port to UWP has been quite smooth, there are still large chunks of functionality that do not exist in UWP. It seems that that functionality will not exist for another 5 years - ant there will probably never be a browser deployment option, or a Linux/MacOS version.
The problem for Silverlight now is a political problem; not an engineering problem. Corporations have realized that app stores make money, and therefore Silverlight is a threat to that model to some extent. If amazing apps can be delivered through a browser, then apps in the store become less appealing. At the end of the day, Microsoft has to make money.
So, if we are honest with ourselves, we have to accept that Microsoft will be reluctant to do this. That does not change the fact that it needs to be done! We are Microsoft developers - always have been, always will be! C# is our language of choice and we're here because that's what we do. Microsoft should open source this out of loyalty to their developers, and they should understand the intangible benefits of open sourcing Silverlight. Open sourcing Silverlight would generate an enormous amount of good will in the Microsoft developer community. It may not translate to $ directly, but it will translate to more developers paying for their annual MSDN subscriptions. So, please Microsoft, get on this!
PS: It makes me warm and fuzzy to read comments here. On most other forums, people belittle Silverlight developers. People here obviously know what's up.