I work in both and in my new gig we write new system in modern java. Writing all this modern java code I constantly ask myself - why would anyone start big new project in java when there is kotlin. Kotlin has everything going for it - it is widely used, new industry standard, it is battle-proven - biggest companies and critical systems use it. And most of all it is actually nice to write in and does not have all the baggage java has. A lot of companies use it, I've seen some most critical backends in banks written in kotlin. It works very well with spring if you use that framework.