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".
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.
maybe? I know I dont have the balls to suggest our enterprise sw migrate to kotlin. I love the language but I think getting management to make such a drastic change can be hazardous if it turns out to cause unexpected bugs that lose millions of dollars :( such is the life of a java programmer
assuming you propose the idea to migrate to kotlin, it would go something like this:
talk to your other developers and see if they feel the same way. get other developer buy-in
propose the idea to management with reasons why it would be beneficial
management now either buys in and approves kotlin usage, or says it's not worth it
if management says yes, you now have like 20 people who have vetted and agreed with the idea. once you start writing Kotlin it's not like EVERYTHING is all of the sudden Kotlin. it's an iterative process, and hopefully you have test coverage. you can even re-use your existing java tests since the languages are interoperable. Assuming you follow a normal development process, the odds of a catastrophic bug coming out of nowhere to cause millions of dollars of losses wouldn't even cross my mind.
that being said, assuming the current code works decently well, management will have no motivation or reason to approve a total rewrite in a new language. it's more likely that they will only approve starting to trickle in kotlin for new projects or features, which even further reduces the likelihood of a catastrophic bug happening.
Kotlin is used as the base of TeamCity's DSL, so I have to use it from time to time at work to configure build pipelines.
But I have never used it to build anything too complicated.
Seems like a massive step up from Java in terms of developer experience. But that's obviously just an opinion.
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 don't know if it's popular but a few companies use it for backend servers instead of using Go or Node.js. It's a language that I really enjoy even if I have never used it professionally.
The best I can get away with is simplifying our older Java when I have to touch certain files. I swear the devs who wrote it must have been paid per line