The DOM wants to be accessed sequentially by a single thread, then we ended up to say single threading is good because we don't have alternatives and because we're using the same frontend technology for backend jobs (v8.) It's a kind of Stockholm syndrome.

Sequential access to the DOM can be ok because we are the only user of the browser. Single processing is not so ok on the backend because there could be thousands of users there. We scale Ruby, Python and Node with multiple processes (I'm doing it.) I'm also developing an Elixir application using Phoenix. The approach is similar to writing Rails or Django code (I never sent a single message in all the application) with the convenience of not having to manage sidekiq or celery for background jobs (they're in the language) and autoscaling.