Yea, yeah, lets forget the potencial big mistake that the last chapter have with some flashy magic and double pages to make the readers happy
In the first place Rust knows the decision he made was the worst one. The author didn't write this part only for drama, or he would have just made it so Rust decision to stay was the best one. It's intentionally written so that the decision Rust made was the worst choice as he portrays his character as more emotional than logical, and brings out his nature as someone who cares a lot for others over anything else, a theme the author has been emphasising since chapter 1. For example, in chapter 1 he decides to stay in the association, even though the logical choice is to leave, simply because of its connection to the previous director who he liked and his old co-workers. The catalyst of the basic department being disbanded and the letter from his friend was the only reason he would leave, otherwise he would have likely stayed under the shackles of old memories. It's not bad writing or drama with no purpose, it's very easy for the author to just make the Ahri say that his choice to stay was the best, but it was written to show him as a character who cares more about his interpersonal relationships over any future troubles.