As opposed to what, threads? They have contributed towards improving software quality? In most languages with mutable state they've been a disaster -- correct me if I'm wrong.
Adding threads in JavaScript would also be a disaster. Practically everything is mutable and there are absolutely no locking mechanisms.
One possible solution to introduce threads to JS is to isolate threads from each other and use only message-passing:
Sorry, but share-nothing threads that can only communicate via (afaics) serialized message passing are no longer threads. There is very little benefit to those compared to just spawning child processes.