> In general, if "orchestrating concurrency" involves guarding access to shared mutable state, then you can't orchestrate it at the callers site.

Shared mutable state is generally behind an API boundary. I'm talking about the exported method set of that API. That is,

    func (f *Foo) Update(i int) (int, error)             // yes
    func (f *Foo) Update(i int) (<-chan int, error)      // no
    func (f *Foo) Update(i int, result chan<- int) error // no