To your last point, this is worse when it's a runtime environment for other people's code... Upstream bugs are extra painful, even in open-source, as getting them fixed and out the door can take time, and your custom branch/fork in an interim may have other bugs, and a "final" solution may not match your own.

Not that it's impossible, but trying to keep certain bits separated is a good thing... I'd love to see proper coroutines/csp in node/js... WebWorkers is similar, but not quite the same, though probably as close as we're likely to see.