People are happily running Rust servers in production using thousands of concurrent threads per second.

At the same time, many others do need more power; that’s why async/await is asked for so often.

I don't doubt that but apparently we have different definitions of a lot. Additionally latency and hardware also matters, I can say that C is doing n things a second while PHP is doing the same but C is running on EC2 micro instance and PHP is running on 2x Intel Xeon Platinum 9282 dedicated machine. C10K problem was not solved by 1:1 model and this is an old problem. C100K+ is what I see in some of production systems I work on.

OK, but, I mean, we've done this experiment, and we found that M:N in Rust was slower than 1:1.

Thousands isn't much.

You can scale up to tens of thousands of threads. But if that isn't enough for your application, then you can use async!

M:N threading was slower than 1:1 in Rust.