User blog comment:B214/DxD Queen tournament/@comment-13032946-20150209034619/@comment-24502975-20150212142746

Rias isn't the source of his power anymore. By your logic she served her purpose already.

Also, things would actually be drastically different if Rose died. How the fuck are they going to re-seal 666? Shiva isn't an absolute guarantee against defeating 666, only their best option.