If only you were truly understand what you talking about:) In fact, Intellij platform is open sourced(https://github.com/JetBrains/intellij-community), many IDE's for different languages built on top of it(like https://intellij-rust.github.io/), as well as Kotlin plugin itself.

And yeah don't forget Eclipse Kotlin plugin, fully open-sourced)

JetBrains open-sourced decades of their hard work, building world's best IDE. And you can benefit from it for free.

You don't even have to pay for webstorm/rubymine. Just install plugins you need to Intellij CE)))