Man... if you don't think node.js on the backend is mainstream at this point I don't know what to tell you. It's not even the hyped-up new thing anymore.
Being not hyped up doesn’t mean it’s mainstream. Most backends are in Java, Go, or PHP. Python and Ruby take up most of those that aren’t. It’s rare to find node on the backend in comparison.
I wonder if part of the confusion here is that “backend” is pretty overloaded. There are backends like API servers, and web server backends (which at Google they call “frontends”!)
I’d guess that Go is relatively more popular than Node for API servers, and Node is more popular for web servers.
And as you note, both are probably less popular than languages like Java and PHP.