If once upon a time MS strength was backwards compatibility, today they are going all over the place. Erratic behaviour such as this does not inspire confidence and rather tells the customer base that you better go with a stable platform. I am not against advancement, not at all, but after forcing Silverlight down our throats and developers investing in it, if they render it useless on the new platform means that what little apps there are on the Windows mobile scene will dwindle down to nothing. Why would I as a consumer even consider it?
I don't think you understand what Silverlight is and what it is/was meant for.
You also don't seem to understand what everything Microsoft has done in app development since then is either.
Here is a very very short synopsis, and will help explain why Silverlight is going to be retired.
Silverlight created as a interactive web plugin, almost a competitor to Adobe Flash.
Sliverlight introduces XAML and new way of building UI, absolutely changes the way .Net developers can build quick, fluid, responsive UI.
Microsoft takes Sliverlights XAML UI design and adapts it to WPF for windows applications.
Microsoft enters mobile market wants to build flexible way of creating apps, uses Silverlight as a way.
Microsoft creates universal apps, ability to create apps without need of Silverlight (remember this tech was designed as a web plugin).
Microsoft creates UWP and wants to retire Silverlight.
Why you say? Well UWP is idea to run apps on any Windows core. Silverlight is built around dependencies on web and old libraries no longer used.
Since Silverlight introduced XAML and every UI tech since then has built off XAML it is not too difficult or resource intensive to make these legacy Silverlight apps into UWP.
Getting rid of old legacy API's and apps allows Microsoft to remove bloat, overhead, and move to a cleaner-faster-more integrated future.