Actually it's not. Universal app means one single application running on multple devices with different interface models. Skype was anything but universal since there were two completely separate applications for Windows 8/8.1 that both had to be maintained separately. It's not unthinkable that Microsoft will eventually release Skype as universal app for Windows 10, 8/8.1, 7 as well as Windows Phone. Until then no modern UI version of Skype.