What if the Soul King was actually held hostage, kinda like Asura's daughter in the videogame, where all the other "deities" became gods because of her power? By Bach killing his "father" he liberates him from his power being used by the shinigami.
For the hostage part, maybe this could explain how Shinigami could "purify" Hollows, since both their powers stem from the same source while the Quincy's have their power apart from the Shinigami's source, which may also explain why they destroy these souls and not "conserve" aka "purify" the soul force.