You can use virtual threads running on a single OS thread and that will work but then everything will be on that one thread. You'll have synchronization but you'll also always be blocking on that one thread as well.
Async/await is able to achieve good UX around explicitly defining what goes on your Main thread and what goes elsewhere. Its trivial to mix UI thread and background thread code by bouncing between synchronization contexts as needed.
When the threading model is implicit its impossible to have this control.
