I think the recognition performance is irrelevant, the failure mode of a model not knowing when there's something it can't recognize is the issue. Most clssifiers will just fail silently and let the toy car drive into a trailer.
Your argument implies that "I don't know what is in front of me" can just be used as a signal to "stop the car", but that still isn't OK (slamming on the breaks on a freeway because a trailer has someone's face on the back of it, for example)
