I don’t disagree with you about the additional inefficiency that is very likely to accumulate as JS adds more and more ‘features’ (via the language, frameworks, or libraries). But as a genuine question, isn’t this reimplementation (or any comparable library for multithreading) required by JavaScript’s position on sandboxing. I would be suspicious of intent if browsers were allowed to spawn any number of threads to execute non-trusted scripts at the level typically seen from more native application code.

Allowing access to native threading doesn’t imply that the API provided by the language is unrestricted. There is a (very wide) middle zone to land in.