Shen Siyuan did not choose adults as the main souls for the Ten Thousand Soul Banner because adults have complicated minds. Even if they are restricted by the rules of the Ten Thousand Soul Banner and cannot harm Shen Siyuan,
they are not mindless puppets and still have their own thoughts.
The human mind is elusive, and Shen Siyuan, in his twenties, lacks extensive life experience and is not a genius.
Though he, as the Banner Master, can control any person, he would inevitably be influenced by their thoughts.
This is similar to ancient emperors who, despite their absolute decrees, were still influenced by scholars and warriors through various actions and thoughts.
Shen Siyuan is someone with a particularly clear self-awareness. From the beginning, he sought to avoid such influences by using a few mischievous kids as the main souls. Although they were not much help to him, they definitely did not impact him significantly, allowing him to maintain his self-integrity.
