Maybe the fame, but I've only recently taken up Kotlin and... I can't say anything bad about it. Yes, Java is catching up, to a degree, but Kotlin is really nice to use if you're on the JVM anyway. (non-mobile btw)
Except for Android, there are no JVM implementations that take Kotlin into consideration.
Java and Kotlin both compile down to .class Java bytecode.
To the JVM, there is no difference
How do you call Kotlin co-routines from Java code without wrapper types?
My main point was that you can use Java code as is without modifications from Kotlin, and that if you wanted you could have Java code in your project and it will work just fine. Of course if you wanna start calling co-routines from the Java part of your project to the Kotlin part, hey that's up to you.