The beauty is that it's not an either/or thing since it's possible to use both in the same app. Kotlin needs less code, making some kinds of classes (like data and event classes) trivial to create since much less boilerplate is required. Moving those from Java to Kotlin is an obvious approach to Kotlin-ize Android apps - I've started one of my apps on that route, mostly as learning exercise, and have been satisfied with the result.
There are many factors and perceptions that matter a lot when we compare any languages, frameworks, or tools. We know that Kotlin is based on Java and you can’t actually compare both these languages due to an influence of one on the other. Many have adopted Kotlin and are enjoying tangible benefits of it, and there are developers who are stuck with Java in android app development. One thing is certain that Kotlin has potential to replace Java language but it may take more time than we imagine.