Right, but I think it's possible to trigger this assert now. For a rather pathological case, consider a client which calls set_async_direct_result on an async which is currently being serviced by a device driver and has not yet reached get_next_device_request.
(And as mentioned, we should probably be calling async_set_initial_status() in async_set_result, in which case this could be triggered much less pathologically by the controlling process dying.)