I work at one of those big tech companies which has a huge number of services in its codebase in Java (you could properly guess the company). We use Kotlin daily for back-end services as a cleaner and more powerful Java, and we've migrated most of our original Java spring/rest projects to be in Kotlin. If you're familiar with Java-based stacks, I think it's fairly trivial to migrate to Kotlin first as a Java substitute before taking advantage of everything Kotlin has to offer, then slowly adopting Kotlin principles.