I don't know about Kwan and Slute being tied. I think many would consider five World Championship titles to two and nine national titles to four pretty decisive.
As for Yu Na, obviously nobody can say now. How long she competes, how her body holds up - those are the unknown variables. To say nothing of Mao 3.0 and Mirai. : )
I agree, Michelle's legacy surpasses Irina's if only because she three more world championships than Irina. But Irina has my lasting admiration for her longetivity - I think her competitive career lasted even longer than Michelle's but I could be wrong. Irina made it to three Olympics. Michelle tried her hardest but couldn't do that. I also admire Irina so much for hitting her peak at a late age and and the end of her career after going through all that hardship with her illness and her mother.
Irina was Michelle's best rival, IMO.
Yuna had already done what Michelle and Irina couldn't, which is win an OGM. She is the first reigning ladies world champion to win OG since Tara Lipinski. Michelle couldn't do it in 2002 and Irina couldn't in 2006.
So in that way, she has already surpassed their record.
I'd say of the three, Michelle is still the only one who can be considered a legend like Kat or Dorothy. But maybe I'm biased because I'm American?
As a figure skating fan, I secretly would love to see Yuna keep winning so even more people in the world know her name. I'd love for Yuna to become a household name even for people who don't follow skating closely, like Michelle and Katarina.
But in reality, of course, Yuna should only keep competing because she loves it. She is already one of the best OG medalists the world has ever seen and a huge celebrity in her home country and among skating fans. So that will never change even if she retires tomorrow.