As much as I dislike him it is interesting to see that even though he makes a point, he is completely wrong.
Bao'er is the legitimate, (most mentally sane) heir to the throne. And Xie Zheng do want to put him on it because the struggle for the throne officially started.
We do not know what happened in details 17 years ago. All we know is that the Emperor abdicated the throne to the current Emperor and is dead now, the crown princess died in the fire as well as Yuanhuai, Qi Min was burned.
But! We do not know what was the starting point of the whole emperor demise plot.
Why did Wei Yan decide to put someone else on the throne ? Maybe he believed at the time that the emperor was no good, we don't know why.
But currently even though Xie Zheng is different, history is repeating itself in a way. He wants to put someone else in the throne.
We do not know yet what will happen. However the Emperor is likely going to die or be exiled, Qi Min is either going to die by Xie Zheng or Changyu's sword or end up in jail. The most likely outcome is that Bao'er will be on the throne but he will be too young to govern as he is 7 years old.
That means that Xie Zheng will likely act as a preceptor for him.
On paper that makes Xie Zheng the new Wei Yan : putting an emperor on the throne, and then playing behind the scene.
But! Wei Yan does not take into account the fact that the soul of Xie Zheng is different from his. Xie Zheng makes decisions based on a path of morale justice and upholding strong values. He does not want power, as he said he just want his own family line, living in the Northwest with Changyu.
Bao'er is also important in that whole power play, we saw the last times we saw him that he had a great understanding of the situation and the stakes of the moment. He is also emotionnaly grounded by his education, mother and Changning.
So, even if Xie Zheng is on paper like Wei Yan, the path that he will take will be immensely different. He will teach Bao'er how to be fair, be just, to uphold justice, and as soon as he can he will step out cause he does not care about power.
This scene ultimately proves that Wei Yan does not understand his nephew and his motives.
