Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130828162355

Good theory Stefan, I can see this as a possible background story for Yhwach.

Regarding Ichigo's Bankai - the old one appeared to be an imitation created by Yhwach-Zangetsu. I think it was a sort of spirit weapon, but appeared as a real Zanpakutô because of the small influence from Zangetsu, that Yhwach-Zangetsu was unable to suppress, and that this is the reason why the old Bankai could not be stolen.

We still haven't seen Shinji, Rose or Kensei release their Bankais and have them attempted stolen, so we don't know if the hollow-influence can make their Bankais unable to be stolen. But Ichigo is different from them since he was born with his inner hollow.

The real Zangetsu appears to be part hollow because of the fact that Ichigo was born with his inner hollow. And because of this, I think that it will eventually turn out that Ichigo's real Bankai cannot be stolen either.