Earlier quoted context omitted.
Electron (WebKit) is much more than a rendering engine though. Sure, you could use Servo in a desktop app, but you’d have to also bundle all the other browser components (a JS engine, for example) individually. You’re basically building a browser at that point.
Servo currently incorporates a network stack, a JS engine, the DOM, JS APIs, an HTML parser, a CSS styling engine, graphical rendering, media parsing and playback, etc. It is not a browser, since it doesn't impose requirements about how to load specific pages, interact with tabs, expose history or bookmarks, etc.
I would love to be able to contribute improvements to the desktop environments I use, but I don’t have the time to learn languages that aren’t applicable to my daily work.