I've found Scala to be a fabulous language for writing > 90% of my app(s). The overhead of Scala's built-in collections, for loops, property accessors/mutators, closures, etc is simply unimportant outside of critical hotspots and provides so much in terms of brevity and expressiveness.
Yes, Scala's lack of looping constructs (break, continue, a real indexed for loop) means that you're better off writing performance-critical things like crypto or protocol parsing code in Java (or if it really matters, C called via JNI). But once you're concerned about performance at that level, every language requires you to carefully consider what your code is really generating, what your allocation patterns are, etc.
Scala is a much more expressive language compared to Java, and the Scala standard library is excellent. Completely dropping Scala for Java when you're building large, complex systems seems like chopping your foot off because your small toe hurts. Use Java or C for the % of code that needs it and Scala for the rest!