What's so important about being async by default to build desktop applications? Offloading slow tasks to a background thread is not at all harder and actually allows you to use more than one cpu core.
Threads avoid the need of callbacks/promises/async/await and they're also cheap and performant unless you need to spawn >10k of them, which seems very unlikely in a desktop applications.
Being async by default is an advantage when you need to handle hundreds of thousands of tasks, all of which are mostly waiting on I/O, which is something that can happen on server side applications, and almost never on the desktop.
In fact, Python is at a disadvantage here anyways because it's not async by default.