By a wide margin, the overall consensus in the JVM community seems to be that Scala will probably never be more popular than it is today, which means, it will never become mainstream. Hopefully, one of Ceylon or Kotlin will succeed where Scala failed.
With that in mind, Play betting on Scala is quite a risky direction indeed.
also, play is not "betting" on scala. Just because it was implemented in scala and provides a scala API that does not mean you can not use the framework from Java (or Kotlin, or Jruby etc.). If you do not care about the scala API, that's fine, use the Java API instead.
I'm seriously asking, not trying to challenge you.
Taken in isolation, these numbers don't mean much but when you start putting all the evidence together, it's hard to make a case that Scala is growing at all.