> I'd argue the opposite regarding tremendous efforts and motivation; dynamic languages were an after-thought for both the JVM and .NET, and on the JVM side you've only seen any attempts to improve performance recently, with invokedynamic for Java 7.

> The investments here occurred long after the VMs were developed.

That's a valid point. It is possible that some entirely new kind of VM could work, that was designed from the ground up for both static and dynamic languages. But I don't think we have any good idea of such a thing!

Until we do know how to do this, replacing the JS VMs in browsers would slow everything currently running. And adding another VM alongside it would decrease speed in other ways (see for example the papers that the Apple dev mentions in the WebKit discussion for why Apple opposes adding the Dart VM into WebKit).