Do-notation is much simpler than callback hell

You could have had the Loom experience 20 years ago by just spawning OS threads. Of course, there's a reason that this was discouraged... threads quickly turn into a nightmare to manage safely, especially when they need to interact.

Genuine question, how is managing cross interacting virtual threads any different or easier than managing interacting threads? I say this and I am greatly looking forward to using Loom in production. It's definitely the correct way to go as opposed to async/await.

It's not. That's the problem with the thread API that Loom is so dead set on preserving, and the big improvement that promises/async/await provide over threads.

How does async/await improve on managing mutable state across threads of execution (tasks/promises/etc?)

Structured concurrency.

Issue with os threads is that their number is limited. Not the issue with communication. Futures are good enough for that, if you need more then structured concurrency.