Earlier quoted context omitted.
"Regress" would be more appropriate, it's taking away useful features. Gimping what an application is capable of should not be the solution to security.
How are you going to secure user against key loggers when there’s a “useful feature” allowing any app to listen keyboard events of the whole windows station? Keep in mind there’re another useful features, allowing users to run an unsigned application from any source, with administrator privileges.
There are permissions for which apps can use my Mic, Camera, Contacts, Call history, etc... Microsoft could easily add one that lets me run UWP apps with a higher privilege that allows IPC with Win32 apps.
As it is right now, you can't even build a UWP app that can talk to a server app running on the same exact machine. Instead, you have to build all of your UWP apps so that they depend on some Internet server.