> In any language with any concurrency support there's a point where you spin up a bunch of threads to do something.
No, that's not true. Think javascript (and I hate javascript): you can do concurrency even though you literally don't have a way to spin up a thread in the browser.
You actually can spin up a background thread in the browser now using web workers[0]. Although, like you mentioned, you don't actually need threads for concurrency. The event loop handles concurrency even in a single threaded environment.
