Isn't BoW kind of a joke because everyone can use it, who's talked to a certain NPC? Isn't the Dark Brotherhood kind of a joke because all they can really offer is BoW and you get it from the first NPC before you are even initiated into the guild?
Everyone having BoW is like a child without a sense of empathy and right and wrong running around with a knife. In-game, everyone is a master assassin. It's funny to wander into a town in Vvardenfell and see all the NPCs are slain and their bodies are lying around.
Why not make BoW only usable for Dark Brotherhood contracts and you get a passive at max level that allows you to use it more widely in the world? At least, this way you have to work for it and suddenly doing the DB questline makes sense.