i believe that with evidence of past games the DB is in danger of going extinct. In Oblivion

It's possible, but I doubt it. I think the Dark Brotherhood is too popular for TES to truly get rid of.

that is not why the Mages Guild no Longer exists.

After the Oblivion Crisis, Magic (Conjuration in particular) was blamed to be the cause of the entire Crisis. as a Result, the Guild (a center of most magical education) was blamed. as such it was disolved and became 2 "splinter-organizations", the Synod and the College of Whispers

There may be lore reasons for the Dark Brotherhood to become extinct but I think it is too popular with the fans for Bethesda to allow it to happen.

They may die because I will destroy them... again!!

but seriously, quite a bit suggests the DB is fading away.