Python has had async for 10 years – why isn't it more popular?

With the newest Python, I can use no-gil, so my threads automatically use multiple cores. With asyncio, even under no-gil, unless I use a base layer that offers parallelism, I am stuck to a single core by default, which doesn't make any sense in a multicore world. In contrast, with Rust's async, there is no such limitation.

The traditional argument against the above assertion has been that asyncio is good for I/O work, not for CPU work, but this constraint is not realistic because CPU usage is guaranteed to creep in.

In summary, I can use threading/process/interpreter pools and concurrent futures, considering I need them anyway, without really needing to introduce yet another unnecessary concurrency paradigm (of asyncio).

You should look into the various Pool classes. Back when asyncio came about, I did a lot of experimenting with various multi-core approaches: https://github.com/rcarmo/newsfeed-corpus