I did this, got approval, wrote a relatively large project in Kotlin only to come back after a few days holiday being forced to rewrite it in Java8 against my will. Left pretty soon after that to focus on startups. Haven't written a single line of Java in over a year, guess it worked out for me somehow.

Why didthey make you rewrite in Java8?

Someone complained that they didn't understand one of the lines of code and somewhere somebody was still using ant to build stuff even though I spent ages writing maven scripts for that project and deleted those ant scripts from git - so obviously something broke since ant didn't have Kotlin compiler in it and that was enough reason for management to have a total freakout about this harmful new tech I brought into the company

Similar story. I implemented POC project with e2e/integration tests in kotlin, cunducted a demo for the team, another demo for managers. Tried to outline safety, conciseness and pragmatic approach. Some people were sceptic, a few coworkers were eager to try but it did not convince management. "It will be better for the project and company to use java." they said. I left that company in half a year.

One of the arguments given to me as well was how difficult it was to hire Kotlin devs - then when they needed Java devs, they hired a Python dev and trained him to do Java. I literally know dozens of people who'd give a left kidney to be able to work in Kotlin