Sure, but keep in mind switching to a new language also entails a huge investment in tooling. There is definitely a higher cost in switching your build infrastructure, tools and IDE plugins to support a new language (nevermind porting an existing codebase) than the few configuration lines it takes to add an annotation processor. At a certain scale, the tools you need might not have even been built for Kotlin, while they have existed for years for Java.
> For many people, this has already happened with Java.
Yes. And, for many others, it hasn't.
Look, I am not arguing that we should all be writing Java until the heat-death of the universe. Whether it is Kotlin or not, I am sure better ways to do things will pop up. But there are good reasons to start even many kinds of new projects in Java today: maturity/efficiency of the compiler, tooling support (including static analyzers, since nullability type systems are not the only feature you'd ever want to bolt onto a language), etc. Over time those reasons will be less, but even then, existing Java codebases will remain, and it will still be worth making them safer. Is not like C, C++ or even Fortran codebases are not around anymore.