Why must the Six Great Masters ascend to sainthood? Would their ascension not pose a threat to him?
The answer to this question is simpler—he must suffocate the last possibility of a Great Daoist Sage emerging.
The only way for this world to undergo its final, perfect ascension is for a Great Daoist Sage to be born here.
His assistance to Jie Yin and Zhunti in becoming Saints is meant to pressure the Three Pure Ones into ascension; otherwise, with two Saints of the Heavenly Dao added to the mix, the Three Pure Ones would lose the initiative and be perpetually suppressed by Jie Yin and Zhunti.
When the Hongmeng Purple Qi appeared, Taiqing was the last to make a move, and it was not to lure out the experts hidden in the world.
In the race for sainthood, whether it was the soul of Kunpeng, or Kong Xuan the Yinglong, they had long been discarded by the Six Great Masters.