Yes but that's geared towards building an API (ofc I can strip Rails however I like, dropping whatever I don't use) but it wasn't my main point - the tooling in the JS world changes rather quickly, now, this would imply lots of changes in main Rails just to support all of those external changes hence the volatility part in my comment.

For example looking at the Phoenix framework - they choose by default Brunch [0] but they also let you implement whatever you want.

It's still opinionated but they make it clear is not part of the core framework.

[0] - http://www.phoenixframework.org/docs/static-assets