User blog comment:B214/Volume 22 spoilers/@comment-82.199.200.228-20160720155513/@comment-1359322-20160720160531

Well a household is only considered "extinct" after all of its members loses their Pure-blood status, for example if Millicas doesn't marry a single Pure-Blood Devil at all, then the Gremory would eventually be listed as an extinct household since Rias and Millicas' children aren't Pure-blood Devil.

In short, we should assume there might be another Lucifer alive, hence their position is retained. No one said that Rizevim is the only son of the Lucifer, or Vali's father is the only child of Rizevim after all.