I'll take a crack at this - coroutines are not about performance. They are for converting callbacks to a single thread of execution that's easier to reason about (which could themselves be embedded in other threads).

Imperative logic is equivalent to a state machine but again much easier for humans to reason about, that's why developers got so much more done in the past with synchronous execution and macros than they do today as we struggle to build even the simplest user interfaces. I'm hopeful that we can get all of the benefits of 1980s cooperative threading but that the decades of experience since allow us to avoid the pitfalls.