Kotlin's fame is mostly due to Kotlin Virtual Machine, aka Android.
So as long Android stays around, Kotlin will keep its relevancy.
Same with Swift and iDevices, especially since Metal was the only thing that Apple still bothered to implement in Objective-C first, with Swift bindings.
Now Ruby, well outside Rails I hardly see any demand for it on the circles I move on.
I've had a few hits from recruiters lately that mention Ruby on Rails. My experience in the Salt Lake City valley was that several successful businesses started with that stack and now it's just sort of "entrenched". Some have been working to move to Elixir, but they still have to turn wrenches on the old (money-making) stack. It's kind of created this microcosmic COBOL dev-like niche. We still need folks who know this stuff. With all that said, I do think there are new startups that still choose Ruby on Rails for its quick time-to-market ability. So demand definitely varies by location.
Maybe the fame, but I've only recently taken up Kotlin and... I can't say anything bad about it. Yes, Java is catching up, to a degree, but Kotlin is really nice to use if you're on the JVM anyway. (non-mobile btw)
JVM is built on Java, as is the whole ecosytem, also doesn't help that JetBrains seems to only care about JVM to the extent it helps bootstraping Kotlin own ecosystem.
Except for Android, there are no JVM implementations that take Kotlin into consideration.
The beauty of kotlin is they don't need to.
I use kotlin on the backend (never did android development). It's amazing. Better version of java than java. Use any and all java libraries transparently (there's nothing to change or do), heck you can even have the java and kotlin in the same codebase (something I haven't really done, but tried it for fun)
Java and Kotlin both compile down to .class Java bytecode.
Suspend functions appear as regular JVM methods with an extra Continuation parameter. Tools like runBlocking make them straightforward to call from Java.
My main point was that you can use Java code as is without modifications from Kotlin, and that if you wanted you could have Java code in your project and it will work just fine. Of course if you wanna start calling co-routines from the Java part of your project to the Kotlin part, hey that's up to you.
There is, but lacks all the nice tooling Objective-C and Swift enjoy for Metal development, it is only there for game developers to integrate Metal into their C++ engines, without dealing with Objective-C++.
The only place C++ has really first class treatment on Metal is a shading language.
So as long Android stays around, Kotlin will keep its relevancy.
Same with Swift and iDevices, especially since Metal was the only thing that Apple still bothered to implement in Objective-C first, with Swift bindings.
Now Ruby, well outside Rails I hardly see any demand for it on the circles I move on.