> Concurrency is hard and if you want M:N thread multiplexing (i.e. WEB SCALE CODE, where application threads aren’t pinned to pthreads) your options today are precisely Erlang, Go, .NET, and Perl 6.

Putting aside the "web scale" jokes (http://www.mongodb-is-web-scale.com/), this statement is still absurd.

Every major language, or at least the ones that matter for backend development, has support for thread multiplexing / coroutines / fibers, whatever. Perhaps not in the core language syntax or standard library. But it's easy to implement with native code, and so SOMEONE has implemented it in a library if the language has an FFI.

Java, and all of the other JVM-based languages in turn, have Quasar (http://docs.paralleluniverse.co/quasar/).

Ruby has support for primitive fibers baked into the standard language (https://ruby-doc.org/core-2.1.1/Fiber.html), and likewise community gems with more robust functionality like Quasar.

Python 3 likewise has this out of the box (https://www.python.org/dev/peps/pep-0492/).

The list goes on and on: https://en.wikipedia.org/wiki/Coroutine#Programming_language....

https://github.com/ruby-concurrency/concurrent-ruby for those wondering. Rails uses it as of version 5.