In many cases, you would get warnings when accessing a var that you don't get when accessing a val. Also, IntelliJ suggests converting to val if a var is never reassigned.
I wish some good designers would lend LibreOffice a hand. Even without reworking UX they could win over more users by being less ugly (compare MS Office start page)
Kotlin has been useful to me and easy to learn. It dovetails so well with exisiting java libraries that no Kotlin revolution (rewrite everything) is necessary. The most notable frameworks are in android land (Anvil, Bansa -- like react, redux). But on the backend, you can use stardard stuff. That's why Kotlin has been well accepted but not made a huge splash.