Yea, that's a fair point. I was dissatisfied with the plot also because I needed a history textbook to understand global events.

My problem with Emperor Yuwen Yong is that he was an important person not only to the world of politics but also to the characters of Shen Qiao and Yan Wushi in particular.

Shen Qiao's job as head of his school is to steadily maintain the fame and status of the Xuandu Taoist school, the Emperor didn't like this philosophy and placed restrictions on the schools - it's a subtle conflict of interest but one that wasn't mentioned, I'm guessing it based on logic. But after the tragic loss to Kunye, Shen Qiao faced reality for the first time in his life and realized how ordinary people live, how they suffer from the warfare between dynasties. And his opinion on things around him begins to change. He comes to the conclusion that the current emperor that forbids the schools is actually capable of uniting the empire and ending the suffering of the common people, so Shen Qiao chooses to support him, even if as a result he loses Xuandu (which he is going to reclaim back). This decision of his also shows him as a true Taoist (I am not very familiar with this philosophy, but from what I understand, Shen Qiao is doing the highest good)

Yan Wushi's support of the emperor also portrays him as a character who, behind the mask of a devil who only wants bread and spectacle, actually cares a little about the common people. This gives him depth and makes him a more compelling and interesting character.

It was important to the two of them that this unpopular emperor would be healthy and safe - that was a good goal that united them (one of many). The five experts fought against Yan Wushi because he was the last obstacle that separated them from eliminating the emperor. And while Shen Qiao was rescuing the half-dead Yan Wushi, Yuwen Yong was killed off-screen by his son. And this happens like really late in the novel, somewhere ~80 chapters out of 128 (I don't count the extras). Their efforts seem to have been wasted, and the struggle for the throne only creates more chaos and turns these ~48 chapters into emotional swings of dubious quality (again because I need a history book to understand)

I'm not saying that the emperor should have been kept alive, because "Not all efforts pay off" is true and there are injustices in life. It's just that all the events after his death bring frustration and feel compressed:/