The advantage of async tasks for structured concurrency lies in task cancellation, which is intrinsically linked to the notion of "task ownership". If you are using an OS thread to offload some task, and then realize that you don't need that task's result anymore, your safest bet is to let the thread run until the end and then discard the results it produces. Other options include adding custom cancellation logic to the thread and remembering to call it at the appropriate time. Nobody checks that you are doing this correctly, which means you may leak resources such as the thread's memory or a TCP connection. On the other hand when using async/await in Rust, the fact of owning a future (i.e. owning the promise that will return you the value when it's done) implies ownership of the task's resources, such as memory, file descriptors, or TCP connections. Dropping the future before it completes means that the task will stop and all resources will be freed/closed immediately, and this is checked statically by the compiler.
POSIX thread cancellation has existed with defined (though complex) semantics for ages. It's a ginormous ugly mess, but it is an alternative to run-to-completion or custom logic.
Anywhere you have an .await in async code you could have a checkpoint in a thread that allows for cancellation. That's the main cancellation advantage - that the author is forced to write those to consume other async functions.
