Why wouldn't they use closures and other new stuff in Java? It's the same like it was with generics - prior to 1.5 we used raw types with casting, but when generics were introduced we started using them instead. When you encounter some older code in your codebase that still uses raw types instead of generics, you just rewrite that portion of code. No problem with that.

I mean, couldn't your point be made about any language that introduces new features? When that happens, do developers decide to rewrite the entire codebase? I don't think so.