The Scala experiment, as you say, seems to be kicking some serious tail, no need to speak of it in the past tense ;-)

Big companies are adopting Scala (Twitter, LinkedIn, eBay, Yahoo, Sony, and so on), not surprising given Spark, Spire, Akka, Play, etc. The Scala language itself is certainly not without problems; at this point the ecosystem behind it is arguably its greatest strength.

Rest assured, if there is a language that will supplant Java on the JVM (highly unlikely in the near-term), it will be Scala (or some variant [Dotty] thereof). Ceylon's got a shot but they're a bit late to the party, and Kotlin isn't even 1.0 yet (Java 8 and 9 will punish that project's adoption, IMO).