Zhai Nanyi understood Qin Hao's intentions upon hearing his words. He intended to drive out the older members of the court and establish his own autocratic rule.
More precisely, to establish an autocracy shared between him and Ye Lingyi.
Zhai Nanyi had neither the strength nor opportunity to resist. Though his family had many members in military positions, Qin Hao's prestige in the army was at its zenith, unassailable, and quite terrifying.
He ultimately agreed with Qin Hao because he saw through the current situation.
The current state was that those who followed Qin Hao would live, and those who opposed him would perish!
If the Zhai Family wished to continue developing, they had to fully comply with Qin Hao's wishes. This may even make them stronger, but it would be impossible to escape his control. However, non-compliance would lead to the Zhai Family's decline.