I write this kind of stuff all the time because parallelizing long-running tasks without dependencies is one of the easiest wins when it comes to wall-time.

But this kind of optimization is somewhat orthogonal to async/await. You don't need fine-grained async to optimize long-running tasks, you could just throw a bunch of closures into a threadpool for that purpose. Async only makes sense when you're interleaving thousands of tasks with readiness/completion based IO.