I keep using it because I'm used to all its warts and idiosyncracies. So I don't know what properly-supported alternative one should use. I just get annoyed how many brand-new fresh-out-of-college developers I meet that use it. They need something better.

Having used both WPF and Silverlight until 2010 (at which point I abandoned the Microsoft stack altogether) I agree, but I don't think the answer is either of those technologies.

Have you tried building GUI apps in Racket? That's the sort of thing I was wishing for when using either Java or .NET to build Windows GUIs.

I've done some academic intro-to-FP stuff in Racket, but haven't really got my feet wet with a non-toy application in it. So the GUI framework is good?

Yup. I haven't built anything of significance in it (yet) but it's proved really easy to learn, and (again, in my limited experience) rock-solid stable and fast enough:

A trivial example:

  #lang racket
  (require net/url
           racket/gui/base
           racket/sandbox)

  (define (menu-file-exit-click item control)
    (exit 0))

  (define frame
    (new frame% [label "Demo"] [height 480] [width 640]))

  (define menu
    (new menu-bar% [parent frame]))

  (define menu-file
    (new menu% [parent menu] [label "&File"]))

  (define menu-file-exit
    (new menu-item% [parent menu-file] [label "E&xit"] [callback menu-file-exit-click]))

  (send frame show #t)

... is all you need to create a basic GUI app with a File -> Exit menu option. And that really is all there is - no resource compilation, no code-behind, no separate languages for expressing the UI and the actions connected to it.