It's taking a long time because they don't want to break existing code, and don't want to loose too much single core performance. I'd say that counts as inheren

It's taking a long time because they don't want to break existing code, and don't want to loose too much single core performance. I'd say that counts as inherent difficulty in changing the language. I'm not aware of other languages trying to retrofit multicore without breaking existing code, but I'm sure if any did other people will correct me and can say how it went. Edit: they're also working on an effect system at the same time, making everything fit into a 25 years old language that's really stable is not trivial.