User blog comment:Xilinoc/Ch. 612 - DIRTY/@comment-25162509-20150122163222/@comment-25889901-20150123024153

Whilst hilarious to think about just how it would be phrased, it's a pretty obvious development here. Aizen's the second-fourth most power being in existance (That we know of), and has a power that directly counters Yhwach's in theory.

Besides, Kubo obviously needs to affirm Yhwach as being the final boss, by having him curb-stomp Bankai'd Aizen. Because we definately need utter proof that Yhwach is more powerful than anything. Except Ichigo because plot.