Even if this were less coupled, there are binary modules within npm that are also coupled with node and v8 in order to build against.
There were some spidermonkey experiments with node, that could run against it, but the modules in npm, specifically platform specific modules (ranging from bson to sqlite and beyond) that are tethered beyond javascript (v8 specific) that wouldn't work.
I don't know that it would bring that much to the table... one could do a bridge similar to what EdgeJS does with node and .Net that could work. It wouldn't be Rhino/Nashorn specifically, but might be worthwhile.
sounds almost like http://openjdk.java.net/projects/nashorn/
Popularity is always a problematic metric for platform evaluation, because the programming community generally has a very short attention span. I would attribute the lack of marketing to the previous license fight, work towards platform stability of version 2.x, and work on the upcoming version 3.0.
> a polygot application framework sounds like the ideal way to use the right tool for the job.
Vert.x is not completely polyglot. It supports many languages, but they all must compile to JVM byte code to work within the platform's event-loop/thread-pool management. So, the Javascript must run on Rhino or Nashorn, the Ruby must run in JRuby, the Python must run in Jython, etc.