You mentioned flight control systems: those are unlikely to be written in Scala, being real-time systems, but if one were, it would not need to be compiled and recompiled on the fly.
Scala's a great language but it's not the right language for every problem. No language is.
For many large, non-trivial systems have several requirements:
- tends to have complex dependencies
- needs fast builds
This was a classic: http://www.amazon.com/Large-Scale-Software-Design-John-Lakos...
Essentially how to design C++ systems so that incremental compiles don't sink you. Developer productivity is important.
Scala is badly failing on this front IMO, it reminds me of the old bad days of C++ in a major way.