"I can fight, but I am not allowed to win!"
Rowan could kill a Throne of a Primordial, and this would not bring as much consequences as killing the manifestation of Light, because it brought the startling purpose that all Primordial brought to Reality to the forefront.
The Primordials had taken the foundational concepts of Reality as their own, and the death of any of them meant that a pillar of creation had been extinguished.
In other words, this was a battle that Rowan could fight, but he could not dare win because the outcome of his victory was too terrible to consider. It did not matter which Primordial he killed; the outcome would always be the same: the end of all things.
Ultimately, this was the most outstanding defense of Primordials: that all of existence depended on them.
