The API contract stood out to me on first reading, too (even before seeing the posts here), and I had the same thoughts about how the concurrency runtime is likely in a better position to provide good error messages than the “Swift userspace” executor code.
Is there potential here for a middle ground solution where the protocol requires a Bool-returning function (or property?) which is allowed (but not obligated) to fatalError?
This would accommodate Dispatch, while leaving a better API design for the language going forward.