Seju is the only one who deserves a happy ending, Sumin doesn't deserve it, but it seems like the majority of readers didn't realize that team gaji gave Seju the best and happiest ending compared to Sumin, bcs Seju completely healed and stopped being codependent on relationships to be happy. Unlike Sumin she's still way too codependent on relationships to be happy, so I would say, she only has a temporary happy ending, bcs I'm sure her relationship with Sungji won't last forever.

The author said "what does the fox say" is a healing story of two flawed characters, but did Sumin and Seju really heal?
Well Seju did really heal, but Sumin? That girl is far away from healing, let's do a little comparison between Sumin and Seju:
- Seju's biggest flaw was that she loved Sumin too much, she didn't learn how to live a life without Sumin, all these years she thought she would just kill herself if Sumin will never see her again, and all of this is coming from the fact that Seju has never received love before from anyone, she always had suicidal tendencies, so basically when Sumin came to her life and loved her, Sumin became the reason why she is clinging to life and why she wanna live, Seju was too emotionally dependent on Sumin. But by the end of the story we can see how Seju was the better person, when Seju decided to move on from Sumin she did it in the right way, she didn't go sleeping with any woman she meets, she didn't start excessive drinking and turn Into an alcohol, all what she did is to decide to never see Sumin again to the point she had to sell her whole company and move out from the city to never see Sumin's face again, it's the thing Sumin failed to do all these years, even after she decided to part ways with Seju she still couldn't do it in the right way, she still went back to work in Seju's company and refused to sell the house that is full of Seju's memories, she even kept pics of her and Seju.
The author said that even if Seju will go through another break up, she will be fine, bcs she learnt how to cherish her life and love herself and to never make her life depends on her relationship state unlike how she used to be with Sumin, basically Seju got rid of her biggest flaw.
- Now let's discuss Sumin's healing:
Sumin's biggest flaws are... well there are many of them:
Sumin made dating her whole personality, like if she isn't dating and in love she isn't happy, through the whole series her happiness and mood depend on if she is in a relationship and in love, like there's nothing that can make her feel happy except being in a relationship, other than that she doesn't know how to enjoy life on her own, everytime she's not in a relationship she's really depressed and in a bad mood.
Sumin is so fkn selfish, self centered, egoist, who doesn't know how to forgive and she takes ppl for granted.
The story has ended but did Sumin get rid of any of her flaws? No, not a single one of them.
Idk how the author can call it a story of healing when they failed to heal the protagonist, they really think finding a doormat girlfriend is healing. The author made it clear that Seju will be fine if she goes through another break up, but what gonna happen to someone like Sumin if she breaks up with Sungji? We already have seen till the very end how Sumin just locked herself in her house being depressed when Sungji cut ties with her, so once they finally break up for good, Sumin will just get back to her unhealthy life style? Locking herself up being depressed, drinking and smoking and maybe even random hooking ups looking for love again?
And for someone like Sumin who didn't learn forgiveness and giving second chances, if Sungji will make a big mistake, Sumin will just close her ears refusing to listen and maybe turn revengeful?
For someone who is selfish and takes ppl for granted like Sumin, we have seen her being the same selfish, self centered person who takes ppl for granted in "will" spin off, Sumin had a bad break up with her lawyer ex girlfriend, once she needs her, she finally remembered her and met her acting all nice and fake bcs she needs her, Sumin even called her : "Sangami-Ah" it reminds me when Sumin all these years was calling Seju "Baek Seju " after their breaking up not "Seju-Ah", but when she needed something from Seju she finally called her "Seju-Ah" then Seju got mad and told her " don't call me that, you only call me that when you need something from me nowadays ", how shameless Sumin can be to just call ppl who she disrespected like that only when she needs them, to get what she wants from them.
Till the very end Sumin is still a self centered and ungrateful person, does she even feel sorry for Sunhwa? How the hell her death just made her think of writing a will for Sungji? Bcs she thought she might die young too? What the hell is this kind of realization? Ofc she did it to feel a little bit better about herself. And you know what is the most ironic thing? Sumin could have never written this will to Sungji if it wasn't for Seju, Sumin should be grateful to Seju during her whole life, Sumin became rich bcs of Seju. Sumin was just a typical office worker just like Sungji, we have seen her when she was working in the company of red hair girl, after their break up, she had a job at another company, the company was small and with low salaries, the team management didn't even exist, until Seju took over the company, it's where the company became really big and popular, and the salaries improved a lot, and Seju made a team management, and made Sumin the team leader who has her own big, pretty and respectful office room, without Seju Sumin would be just a typical office worker with a low salary, even under Seju's company if she wasn't the team leader, her salary wouldn't allow her to even buy a car, I remember when Sumin said she would like a car for her birthday then Sungji said that's costs a 10 years of salary. Even when Seju disappeared from Sumin's life, Sumin still enjoying the privilege of a comfortable life to this day bcs of the high position respectful job Seju once gave her, she should kiss Seju's feet seriously, bcs without Seju she wouldn't be capable to write Sungji this will. But ofc she's still too ungrateful to everything Seju did for her.
In conclusion, Sumin's character didn't heal and she didn't have any character development despite the author saying it's a healing story of two flawed characters, Sumin is still as flawed as ever, nothing changed, the only logical ending the author could have made to show it Sumin's healing is:
1/ if she forgave Seju, which means she learnt forgiveness and appreciating ppl's efforts, especially since she cheated too, but her getting back with Sungji even though she sees cheating as a really bad thing, instead of realizing how much Seju endured bcs of her after being in Seju's shoes if Sungji didn't take her back, all of this plot did nothing, but adding one more personality flaw to Sumin which is hypocrisy, she ended up the biggest hypocrite ever, you are welcome.
2/ Sumin should have ended up alone, to learn how to live a happy life on her own, instead of being too emotionally co dependent on ppl to find happiness and emotional support, this's was her biggest personality flaw, that she can't be happy without a doormat girlfriend on her side, whenever she's alone and goes through a break up she's just depressed locking herself up. Not only that, how ridiculous from Sumin to say to Seju that she would like to love someone earnestly to the point it would feel like dying or falling from a cliff if that person leaves her. Basically Sumin had zero intention to have a healthy relationship, she doesn't realize that life doesn't revolve around dating, she doesn't realize that she can find happiness on her own, she doesn't realize that breaking up is not the end of the world, Sumin×Sungji showed us many signs of how unhealthy their relationship is and how flawed it's, bcs Sumin is still too co dependent on a romantic relationship to be happy, Well I have nothing to say except good luck for her once they break up, bcs I'm 100% sure that their relationship can't last in the long term, it's the most shallow relationship ever. Sumin Good luck with your break down and getting depressed all over again, I'm not complaining, bcs she deserves it, after all the things she did to Seju she deserves to be miserable, how ironic that Seju is living in a healthy way got rid of all of her flaws but Sumin gonna be in the same shit over and over again after every break up.