I feel like most of the comments here are ignoring the fact that this is a straw man proposal, as the blog post emphasises several times. They aren't proposing

I feel like most of the comments here are ignoring the fact that this is a straw man proposal, as the blog post emphasises several times. They aren't proposing to add threads to JS in exactly this manner. They're pointing out that it would be technically feasible, for WebKit at least, and wouldn't add any unnecessary overhead. The specific proposal is just a starting point for discussion.