His section "There is an escape" answers this criticism: "The nursery object also gives us an escape hatch. What if you really do need to write a function that spawns a background task, where the background task outlives the function itself? Easy: pass the function a nursery object."

He goes on to explain a bit more, but one of his points is what really ties this abstraction back into the goto discussion: "Since nursery objects have to be passed around explicitly, you can immediately identify which functions violate normal flow control by looking at their call sites, so local reasoning is still possible."

> A lot of the value of concurrency is in background operations. These simply can't be tied to the duration of a function call on the dispatch thread.

I think they can: applications with background threads would have an outer-level nursery at whatever "main" is for them, and that nursery would be passed into whatever function needs to spawn a background thread.