> there's no guarantee that such a subset will be maintained

There's no guarantee about Java either. This class compiles with Java 7, but not with Java 8:

    class SampleClass {
        static class Baz<T> {
            public static java.util.List<Baz<Object>> sampleMethod(Baz<Object> param) {
                return null;
            }
        }
 
        private static void bar(Baz arg) {
            Baz element = Baz.sampleMethod(arg).get(0);
        }
    }

Like Java or any other good software, Scala will try to maintain backwards compatibility (particularly source compatibility).