Kotlin is not the future of anything. They got the Android boost and still, two years later, all they have to offer is a molasses-slow improvement pace and a shitty dev experience in their signature IDE (which also happens to be made by the same company).
Last time I tried to create a Kotlin project in IDEA, it couldn't provide type information on hover. The Kotlin dialect of Gradle was barely supported enough to be called a "third-class citizen". It's super embarrassing.
Yeah, Jetbrains marketing wants you to think it is the other way round, but actually tooling support for Scala is ahead of Kotlin by far, except maybe Android.
Scala is not tied to a single IDE, it works great in VScode, Eclipse and Intellij and anything else that supports LSP, which is an open standard. And it works really well - including type inference and very advanced implicits stuff.
And recently Scala got excellent incremental compilation support by Bloop from ScalaCenter, which not only blows Kotlin out of the water but even Java with Gradle. The last year I develop purely in Java and I use Scala bloop for day-to-day development, because I couldn't stand multi-minute Gradle compile times. Getting incremental compile times counted in milliseconds (!) or single seconds at worst is something very hard to give up on.
We're writing new apps in Kotlin instead of Java, works fine for us. shrug Vert.x , Ktor apps, Kafka streaming apps. IDEA handles it absolutely fine these days, I suspect you last tried it sometime ago.
Can't comment on the Gradle stuff though, I vastly prefer Maven.
Vert.x is doing us well for high volume apps, Ktor is early stages but looks alright also. That said, haven't benchmarked identically feature compete implementations of the same app in both, so don't really have the metrics to properly compare, but so far at least, it seems comparable.
I would have said that a year ago but I'm not so sure now. I actually think
Kotlin is at risk of suffering the same fate as Ruby and perhaps Swift:
so stereotyped into being viewed as a single domain language to the extent
it gets ignored by the mainstream. To some extent Groovy suffers it as well -
as soon as you say it people think either Jenkins or Gradle and recall their
last nightmare debugging session trying to reason about its crazy dynamic
behavior, all the while blind to the fact that it has huge general applicability
to all kinds of application development. I even think this is starting
to hurt Python where now everyone thinks it is for data science.
I'm not so sure. I don't see it being used on the server side a whole lot, it seems to be more or less synonymous with Android dev -- and Dart/Flutter is taking big chunks of that space now and growing rapidly.