He doesn't. It sounds like you haven't even done the quest. Nazir and Babette meet up and take the night mother to the Dawnstar Sanctuary. If you let Cicero live, he never leaves Dawnstar sanctuary. It's one of the faction quests in Skyrim that makes sense.
Sounds like your not paying attention or
you haven't done the other quest. I'm talking about what happens if you decided to kill Astrid when she abducts you and destroy the dark brotherhood.
Nazir is dead killed along side everyone else in the sanctuary at the hands of the Dragonborn. The only survivors are Babette(who wasn't present) and Cicero who never even arrived. Originally when Cicero came to skyrim he hid at the Dawnstar Sanctuary only later deciding to move to the Falkreach Sanctuary to teach Astrid the error of her ways.
But you rebuild it with barely much more than that.
We do it with a lot more than that, first we have Nazir and more importantly killing Titus Mede II proves the Dark Brotherhood is back and powerful again.
What it really relies on is the Night Mother. If we assume that she doesn't talk to anyone else, if the Dragonborn slays the DB, then yes..they are pretty much done. If she chooses someon else.... it's very possible they could rebuild.
And how long will that take? eventually people will stop doing the Black Sacrament as no one answers.