I've been monitoring adoption for some time. There was a (roughly) three+ year period where Groovy was the fastest growing language in the world. Scala has seen more growth over the past year, but IMHO it will never challenge Groovy. Why? Functional languages have been around for years and have never owned more than a niche market due to their complexity. Groovy is more readable than Java, and much more readable then Scala or Closure.

Remember COBOL dominated the US for many years due to it's simplicity. PowerBuilder beat a more comprehensive competitor (SQL Windows) due to it's simplicity. It seems likely that functional languages may find a niche where they fit well (e.g. business rules) but languages like Groovy seem most likely to achieve wide adoption. IMHO of course!