Threads are driven by the OS. Something needs to drive couritines, so there's no way around needing some (even rudimentary, like in embedded) executor. But to be a versatile and universal systems language, Rust can't just build-in executor into a language.

I think that stackless coroutines are better than stackfull, in particular for Rust. Everything was done correctly by the Rust team.

Again, this is all fair and good, as long as people understand the tradeoff and make good technical decisions around. If they all jump on async bandwagon blind o the obvious limitations, we get where Rust ecosystem is now.