I think I agree with the general sentiments here. Kotlin would make a great first language if learning resources are positioned for beginners. There will come a point as a beginner progresses they will need to learn Java to advance their knowledge. Navigating Java libraries and their documentation, the nuances of the JVM environment, and getting help from a broader Java community all would warrant a need to learn Java.
What will be especially interesting is to see what happens on Android. I see a growing number of Android articles covering Kotlin instead of Java. When new Android developers are faced with a choice of learning Kotlin vs Java, that will be a major shift and we should start seeing Kotlin books for beginners. And yeah, some argue Java 8 coming to Android will marginalize Kotlin. But I switched to Kotlin after being a fan of Java 8 for a year. As some people said about Scala, Java 8 might actually help Kotlin as people are introduced to functional concepts.