I'm not against Kotlin, but seemless interoperability and the ability to have both in the same codebase simultaneously does not, in general, mean you can have a

I'm not against Kotlin, but seemless interoperability and the ability to have both in the same codebase simultaneously does not, in general, mean you can have a lot of your devs skip learning it. In fact, in my experience, projects that were using two interopable languages meant you had to be a lot more of an expert in both. The cognitive overhead of flipping back and forth when just switching files was painful because the two languages were just similar enough. If you had some very silo's portion of the code that you could leave your Java people on, then it's possible, I've just never seen that in practice.