I would be intrigued to see someone try to create a modern dynamically-typed language or interpreter written from the beginning to be threadable There is Cloju

I would be intrigued to see someone try to create a modern dynamically-typed language or interpreter written from the beginning to be threadable There is Clojure, is dynamic and allows for multithreaded programming, courtesy of the JVM. The JVM being a very dynamic system is a good target for new dynamically typed languages. But as you said, they are a hard sell today not only because of performance but more because of static typing.