Now, if you could reduce the overhead of OS threads by using lightweight processes instead ... Wait a moment, isn't that what Erlang does? Well, project Lumen might help you out on the JVM at some point.

With NIO there is no OS overhead like context switches because you only need one thread per core as long as everything is async.

Erlang is single threaded unless you copy memory between threads.

The only step left for Java to implement (after io_uring file stuff) is user-space networking.

The Beam VM handles memory copying, afaik. It is very much multi-process. It has very lightweight processes, of which you can spawn thousands. Those are then run in as many OS threads as makes sense and are available to the virtual machine. It is unclear what you mean by Erlang (the language) being single threaded. The VM easily runs on as many cores as you give it, probably by using as many or more OS threads.