I have friends who work at the biggest bank in Latin America, where most backend stuff used to be Java. Nowadays all new code is written in Kotlin.

Loved using it when I took a brief stint as an Android dev at my company. Later talked to my tech lead to see if he was open to me writing future backend developments in Kotlin but he said it would be too much unneeded work to get the entire team to learn a new language to keep the backend maintainable.

I don't use it right now, but two years ago I helped a team incrementally adopt Kotlin in a ten year old java/spring/mybatis codebase. We didn't have any android experience and in the initial few months mostly used kotlin as a better java, avoiding features that would prevent us from switching back to java if needed.

But it worked pretty well - we didn't face much resistence from people experienced with java because they could still continue to benefit from their jvm familiarity, and the language was approachable to new folks who joined us. It also helped that we could just copy paste java code into a .kt file and intellij would convert it to kotlin.

We didn't venture into kotlin's js/native targets but for jvm it worked out great for us.

I've been using it server side (haven't touched android for ~12 years) for almost 7 years now. It's fantastic.

Last job, we started writing mixing bits of Kotlin in an otherwise mostly-Java in a monolithic Spring-based service. Good experience.

Yes, I write SpringBoot microservices and IntelliJ plugins using Kotlin. Any new code is Kotlin, but there is still a ton of Java, which I don't consider "legacy", since it works, and if I can sanely add Kotlin when necessary, I don't see the need for "full rewrite".

You may get more traction by asking the Kotlin community

Lots of people do, just look at all the server libs for kotlin.