> Kotlin <-> Java interop is a totally optional topic you could have skipped over

This is the same place F# has been stuck in. It’s a great language on its own, but you can’t just use F#. Every F# must also do C# interop. It’s too 100% optional in theory, but never in practice. The best CLR/JVM libraries for anything are Java/C# ones. You need to interop with them to develop practical Kotlin/F# applications. You can limit yourself to the Kotlin/F# ones, but then you’re artificially limiting yourself to experimental libraries at best. You will find yourself needing a charting library, a DNS library, an SMTP library, an AWS SDK or a rabbitmq SDK. The best ones are gonna be Java/C#. Yes, you can always find a random GitHub repo for a “Kotlin-native X”, but the Java X library is a thousand times more mature, stable, performant, feature rich, etc. Same problem with F#. And the “glue” code is so “straight forward”, why would any one bother?