Random thoughts: I get that Coates has problems with a Monarchy and it looks like his goal is to change the government of Wakanda. And I get that in order to justify it, T'challa has to shown as a failure. If he is a strong, successful monarch, then the people's justification for a change is weakened, especially in the minds of the reader. It makes the people look ungrateful. Now, from Doom's attack on T'challa that lead to Shuri becoming BP through to Hickman has given him in-story justification. So, I get that he can't make T'challa look successful and strong now, if his goal is to bring the readers to the point that they believe altering the monarchy/government is the right step.
The thing is, he could have got there, without making T'challa looking like a failure. There is another other in-story justification for the change that is common place in comics. You see, he is, despite comments to the contrary, a regular super-hero, too. That is, he puts the good of the world over the good of Wakanda. He is more warrior than warrior king. Yes, he is more king than Thor is, but just as Thor is constantly putting earth over Asgard, T'challa has spent a great deal of his career outside of Wakanda. So, Coates could have gone that route. He could have wrote him as the king who would rather be on the front-lines defending human life, than on the throne taking care of legal issues. Then you can have him successfully taking down the forces of evil, but it keeps him so busy that the administration of government slips from his control. Then T'challa looks even more heroic by realizing this and accepting change in the government so things could run smoother.
BTW, I'm not saying this is the route I would take; I think there is nothing wrong with the current government of Wakanda and BP's place in it, but if this change is his goal, he could have went at it another way.