The most crucial point was that the destructive power the enemy could wield when they joined forces was simply beyond what Zhou Tian could resist. If he did not make some changes in the face of such strength, he might not even need to do anything else before the three-headed Divine Beasts together would just kill him outright.
In order to cope with the threat from those three-headed Divine Beasts, Zhou Tian first needed to address the issue of the domain power they wielded.
Although having more domains wasn't necessarily stronger, if one side had more domain powers, they would still be more advantageous when joining forces to act against the enemy.