I'm not necessarily disagreeing with you (static typed languages are going to be faster on the JVM), but JRuby - partially with the help of invokedynamic, added in Java 7 - has come a long way performance-wise since Twitter began their rewrite.
I wonder if they would have done anything different, had JRuby been where it is now?
JRuby, even when using a lot of Java based libraries, doesn't get you the performance boost you need — it is just incremental over Ruby and even then only with some code.
The truth is, Scala is actually helpful with the architecture they came up with AND its great that it runs on the JVM.