User blog comment:Xilinoc/Ch. 595 - Rubb-Dolls 2/@comment-493809-20140906232130/@comment-493809-20140907010527

As an example of what Yhwach might be capable of:

The Y brothers were born with abilities that allowed them to imitate the power or the memories of someone, without the appearance. Yet both of them held the moniker of Y and were able to take on the form of the person they imitated. This may be a result of them developing the ability, or it could have been gifted to them by Yhwach as the power of The Yourself.

There may be a distinction between everything the epithet wielders used and what Yhwach gave them. At the very least, he gets some of it. At the very most, everything used by a particular Quincy came from Yhwach, and is going back at some point.

Super-strong heat-beam lightning-strike uber-scary gluttonous zombie iron-skinned Ditto loud-mouth experience-gaining hero as the final boss.

And the Hollow thing is a good point. It could be that the Quincy powers purify them to a degree, that Yhwach is immune/resistant to the corruption or that the nature of his influence is itself the reason for the conflict, or simply that the destruction is so complete that the typical Hollow is assimilated purely as reishi.

Possible theory: Yhwach and the Soul King are a type of being responsible for the existence of the different types of souls, and there exists a similar being for Hollows. The reason why the Soul Destruct process is ended by taking bits from all types of souls is because it allows for a balance close to the state of the proto-soul.

Again, largely just theory.