In Pei Xuanjing's view, all living beings in this world inherently have a nature of self-protection, and no sentient being can avoid it.
If the Witch Deity Teaching is as legendary as the rumours, then the old artifact suppressing the Witch Deity Teaching is twilighting, and so the first thing that the Witch Deity Teaching needs to do right now is maintain their status and not let themselves decline. Given they grasped the means to improve the odds of a cultivator of the Spirit Realm Eightfold to ascend to the strongest Nine Realms of Spirit, it would just be their instinct to let the cultivators within the Witch Deity Teaching ascend. Why would they help Wu Shijiu?