> It's one of the few languages where you can mix static and dynamic typing

In theory, but in practise virtually everyone uses the dynamic typing only. Groovy's really only used with Grails and for testing Java classes.

> Android, running Groovy with CompileStatic enabled by default would make a lot of sense

Unlike other statically-typed languages, Groovy's CompileStatic code was written by only one person and having it be the default would expose all the bugs.