Silverlight is DEPRECATED Perhaps that is why not to use it?
This Article from 2014 should give you an indication of where 8.0 now sits
Here you will see Paragraph 2
Microsoft announced the end of life of Silverlight 5 in 2012. In 2013, Microsoft announced that they had ceased development of Silverlight except for patches and bugfixes. Microsoft has set the support end date for Silverlight 5 to be October 2021. Silverlight is no longer supported in Google Chrome since September 2015. Since Microsoft Edge does not support plugins, it also does not support Silverlight.
So I really do not know what more to tell you regarding Silverlight [Trust me I used to be a fan of it but now it sits in the bin with Expression, Money and Encarta among other discontinued projects...]
For your other question:
Do people not do any research these days?
Here is a useful website for you...
As you can see, it would be pointless to develop for Windows Phone 8.1 as well since again unlike other platforms, Windows Phone 10 will be another free upgrade for Windows Devices [Some lower end handsets may not get the update however, but do you really care for a small percentage?]
It is not like it is not similar to any other platform, Apple killed EVERY application when they enforced x64 support, which meant if your app was 32bit it would no longer be allowed on the store [I could be wrong about this one but I am pretty sure that is what happened], and don't get me started on Android.......
So I would evaluate your development choices before you embark on a project that when completed has no platform let alone no audience...
I can see you have fixed ideas so I will leave this thread now, but good luck to you.