I am also worried about the concurrency side of Kotlin. Since Roman Elizarov, the architect of the coroutine / flow implementation, left JetBrains two years ago, it seems to be stagnant. Looking through the tracker conversations I almost get the impression whoever inherited it doesn't know where to take it next.

That's a whole huge can of worms, too. Coroutines and the suspend keyword were a great innovative feature at the time (very clever implementation on the JVM and good API design given the limitations of the implementation and syntax design constraints), but now that Java has so-called virtual threads, you don't "need" Kotlin for convenient(-ish), simple(-ish), scalable, structured concurrency like you used to, either.

One could rightly debate over whether Kotlin's coroutines design and APIs are better than Java's virtual threads for writing asynchronous code. But, at the core, the story used to be that Kotlin had coroutines and lightweight structured concurrency "built-in" (with a blessed first-party helper library for the actual concurrency part) and Java did not have anything that accomplished the same goals. Now it does.

Vsevolod Tolstopyatov (https://github.com/qwwdfsad) is the other brain behind Coroutines, Concurrency, and Atomics in Kotlin, and he's still very much active.