The reason why Kotlin is gaining marketshare is specifically because they aren’t chasing after things like HKTs. Kotlin is squarely going after the better Java market and not the advanced FP one.
I think Kotlin's design - adding lots of ad-hoc support for specific use cases but without the underlying general constructs that make them coherent - will come to bite them as and when the language needs to evolve over time. (Indeed to a certain extent it already has, as Java 8+ adopts Options which don't play nice with Kotlin's ?. etc.) I guess we'll see how things look in a few years.
I was excited about Kotlin at a first glance but came to feel this way as well, after looking into it a bit more.
? actually should play nicely with explicit Option types, they do in C# anyways.