One should indeed consider many different parameters when choosing a programming language. At least for a long-term, commercial project. How is the community support? How quickly can you onboard your own team? How easy is it to find new developers to work with it, either by onboarding them or finding people who are already experienced?
A programming language might be ‘cool’ but if you can’t find people to work with it, it’s not a wise choice.
That doesn’t mean we should be stuck with Java and disregard any other language though.
Scala, for example, has been around since 2004 and it’s being used in many big-scale commercial projects, especially in big data. Its community support has been rapidly growing and it’s #2 on Stack Overflow Survey’s list of Top Paying Tech in US. #1 is Spark by the way, which was written in Scala. ‘Top-paying’ may not be a good indicator on its own, but Scala is steadily growing in all other categories as well. So, I don’t think it’s simply a toy for cool kids. I think it’s here to stay.
Kotlin is relatively new. But it’s backed by JetBrains, who create the most beloved IDEs. That probably means that as long as Kotlin lives, it’s going to have a solid IDE support. And it’s being adopted by the Android community, especially after this year’s Google I/O. We’ll see the results of this next year. That doesn’t say much about its usage in the JVM, nor should one jump right into it and start writing every backend project with it. But learning Kotlin is pretty easy and it paves the way for learning Scala, since they are very similar in basics.