This is a reason that is brought up once in a while - but even after working 10 years in the domain of high-concurrency services I've never seen compelling data

This is a reason that is brought up once in a while - but even after working 10 years in the domain of high-concurrency services I've never seen compelling data that shows clearly whether stackless coroutines are more efficient than stackful ones. People unfortunately rarely write applications in both approaches to tell. While the "stack is optimally sized" argument exists, it might not always be true: E.g. implementations could require far more memory being required for the "virtual" stack than what is actually required due to implementation challenges. That for example applies in various situations in Rust. Then a more classical stackless implementation which allocates state for each callback on the heap (like if you manually write boost asio code) which have quite some allocation and memcpy churn. And besides that a "virtual stack" might be more fragmented and less cache friendly than a contiguous stack, which also impacts efficiency.