User blog comment:Xilinoc/Ch. 565 (Mangapanda is run by lazy barnacles)/@comment-2220112-20140122223213/@comment-26420452-20140123063852

I think that you misinterpret what Uryū being named as Yhwach's successor means. Yhwach did so because that would arouse suspicion among the ranks of the Wandenreich, make people look closely at the "next emperor" and make it impossible for Uryū to defect.

If Yhwach had granted him exactly the same powers as his, Uryū would've started going a bit blind by now. I can't say what precisely the bad implications for having a Schrift are, but anyway from the dialogue, we get that now Uryū's soul is entirely bound to Yhwach and it's pointless for him to try to leave.