I had this very same problem - even if I created a brand new Windows 10 XAML app the XAML designer stubbornly refused to show up, with no error message. I tried creating a WPF app, and the designer appeared - so it must be a Windows 10 thing.
After hours of googling, and getting nowhere, I started 'tinkering' with my project properties.
I noticed on the project property page there is a section called 'targeting'
My settings show that a minimum windows 10 version was build 10240, and the Target version was different at 'Insider Preview 10563'. I changed the target version to the minimum version and BLAM!! the XAML designer coughed into life.
I have no idea what that was all about, but at least I can get some work done at last
Very interesting. Thank you for sharing, Dean.