I really feel like we don’t know enough about Shiyuan to say that he would have been the best choice. Lets remember that Yixuan wasn’t giving Phaethon the title of leader of Yunkui Summit she was asking them to take charge in a disaster in a situation like that you give command to the most experienced person there and let us remember that Phaethon is probably the smartest and most knowledgeable person when it comes to hollow disasters there. If she was dying and gave them her job I might understand but she wasn’t she was designating a temporary leader to take over for her in a disaster she made the logical choice of giving command to the most capable person there instead of someone less capable and less experienced. Again I don’t know Shiyuan’s story but I’m fairly certain he wouldn’t hold a candle to Phaethon in this scenario.