Make me thinking what is the point of paradise?

I think that the popularity of One can be due to the fact that, tho it's just my observation and I can be wrong, but for a majority of people who read wdtfs Seju was either their favourite character, or at least they liked her similarly to other characters (for instance, there are people who are team blonde, but still say that Seju deserves to have a happy ending and thus are still invested in reading One). Also, it's not a secret that it's been years since wdtfs started and during this time Team Gaji's writing and drawing has been improving constantly, so the way I see it a relationship between Seju and Won also, just like you said, develops very naturally and I'd arguably say that its beginning is way more convincing than the beginning of Sumin x Sungji. It's also really heartwarming to see a character that's been through so much drama and pain to finally be able to smile and act like herself and if you combine it with a really nice art then it can also be an explanation why so many people are drawn to this story.

When it comes to what you said about Paradise, I agree that flashbacks from main story already gave us a lot of information about past of Seju and Sumin - enough to make us emotionally invested in life of these characters, their current relationship, heck it even resulted in many people rooting more for Sumin getting back with Seju instead of moving on with Sungji, but maybe Paradise wasn't there for the same purpose?
Cause the way I see it, even tho these side stories can work as stand-alones, after all they are a part of a coherent story. And if you read Paradise, you see that it starts with Sumin being occupied by her thoughts when Seju is finally moving out and ends with Seju waiting for her flight, with a surprisingly calm expression. To me it just indicates that both characters were reflecting on their relationship, mostly its ups rather than downs and Paradise was just a result of this "mutual reflection". I'd actually say that Paradise was more important for Seju's character than Sumin's, because at the end we see her going back to the very first memory of how these two met and realising that they used to love each other, no matter what people kept saying and no matter how ugly their relationship turned out to be later and with this though in mind Seju is able to acknowledge Sumin and thus her feelings for her as a part of her past and finally move on - not hating Sumin, but also not really loving her anymore as well (contrary to how the main wdtfs story ended with her saying "goodbye, I love you", because judging by the length of her hair, there is a distance of a few months between the end of wdtfs and the end of paradise, so Seju had a lot of time to reflect on her feelings) which then gives her a nice opening for everything that is about to happen in One, including her new relationship. Moreover, after all Team Gaji created such a complex and emotional relationship between these characters, so I wouldn't be surprised if they just wanted to show us how it all started between these two, how they met, how they used to treat each other as friends and then girlfriends which is something that flashbacks from the main story were able to show in a much smaller percent.
Tho of course, this is just the way I personally see it, but I think that Paradise was an important reflection for both Sumin and Seju and even if it's focused on relationship that we already understood, it definitely offers a more in-depth explanation of it, even for such things as development of Seju's possessiveness or how forgiving Sumin could be back then. Also, Team Gaji is not necessary the one to be solely focused on pleasing their audience - after all, they take their time to make a side story about secondary character like Nammi and episodic characters like Dawoon and Sangmi that people care less about, but they go for it because they care about characters they created and the stories that are planning to tell and it can be the same for Seju's and Sumin's past

As for the rest of your comment - I agree with everything! And I think that One will be way longer than Paradise, there's a high chance that it will be the longest out of all side-stories, cause it has so much material to cover - first, the exploration of a completely new characters, then the exploration of Seju's personality when she's not occupied with Sumin, relationship between Seju and Won, possibly any dangerous situation that is about to happen in this villa, combined with Seju's family that seems to be still relevant. If I had to guess, I'd say that One may have around 30-40 chapters? That is, of course, if these things are all going to be explored

A final goodbye to the relationship between Seju and Sumin, and for me it was necessary because they needed a better closure for their relationship and Paradise did a better job than the main story. I loved the final scene with Seju leaving all about Sumin behind her, even the farewell note, and heading alone to an unknown future, it was very symbolic. At the same time the farewell note and that Seju and Sumin remember only the good times about their relationship gave me a little hope that they can be friends in the future.

Also Paradise is like a link that chronologically connects the main story with One.

I agree with you. It’s necessary for us to know more about how Seju found her first love. And how that past leads her to the new start in Jeju island to meet her second and the last love . Actually in paradise , she mentioned jeju island and swimming. And that’s all happen now

I agree with you - Paradise gives both characters and readers a way better closure to a past relationship of Sumin and Seju. Because, after all, main story is focused mostly on Sumin, so she and Sungji, as a main couple, will occupy the majority of a plot and thus the ending to Sumin's and Seju's relationship (with Sumin crying because Sungji left her and Seju finally saying goodbye, while at the same time still confessing her love to the only person she's ever been in a relationship with) definitely fits bitter and more dramatic parts of a story, but doesn't give any feeling of being complete - which is something that Paradise does very well
Also, personally I would have trouble finding first chapters of One as believable, if Paradise didn't exist in a first place. Not necessary because of the fact that Seju at the end of a main story was still around Sumin - after all, there is a distance of few months between the end of a main story and the start of One, which gives a plenty of time to move on, but because the way Seju acted at the end would never be balanced with any image of her coming to terms with finally leaving her past behind. Cause imagine Seju slowly becoming interested in another girl, when the last crucial moment when we saw her was while she said "I love you" to Sumin. I think that what Paradise and (as for me) the most important chapter of it - which was the last chapter - do is they show that Seju is ready to move on, just judging by her somewhat peaceful expression and the fact that she just left behind a note that Sumin gave her, instead of still holding on to it desperately

Waw,, your reply is really long, i like it btw makes me excited to have a discussion,,

About the art, yeah, i remember the first time i read wdtfs i didn't really like the art, the story is kinda hard to understand, and suddenly there were a hard core sex scene that seems meaningless, i didn't favor all of it and put off to read it after less then 10 chapters for a few months,,
Until i decided to try to read it and finally got hooked,, the art keep getting better and better, sometimes i felt like i was too invested to this manhwa, i felt like the characters are too real and they have emotion and alive, hahaha,, that's how you know how good the authors are,,

Well if team gaji do make 40 chapters for One, i think they would be the only author who make side story that almost half chapter of the main story (most of the author i know only make less then 5 chapters for side story),, i won't complain at all tho, but realistically i think they would make twice as many chapter than paradise to 30 chapters for enough to cover won's background, make seju & won story believable and have a strong bond, seju's family, etc,, here'a to hoping that team gaji would make more than 40 chapters,, at this rate i swear it would be better for team gaji to make a whole new series just for seju

YES! SAME! At first I didn't get into reading wdtfs, cause at that time it wasn't finished and there were so many people saying that they think it may turn out to be some sort of a threesome story and I was just like,, man, I don't have time to read over 80 chapters about that :') Then when I found out that it's actually sort of a drama/love triangle situation I wasn't interested either, cause I don't usually like this trope (tho Team Gaji actually executed it very well!) and, just like you said, during first chapters art is not as good and it seemed pretty stiff and story in general starts very slow, but after few chapters it easily makes you suddenly invested in live of these characters

You are right - 30 chapters is probably enough, tho I also wouldn't mind if it was a bit longer. And I'm pretty sure that One may be the longest out of all side-stories, cause when you think about it, after all Seju was a crucial character in the main story, she went through a lot and so there's a lot of topics that are probably going to be discussed in her own journey. Besides, in her case, it takes a while to fall for someone again and Won as well is either a bit oblivious, or holding herself back so it will take more chapters for them to end up together. Tho, after 7 chapters they already start to be getting closer, so you may be right, 30 chapters could be enough!

And yeah, sometimes I tend to write long essays in comment section, but that's just cause I really like writing in general and I'm happy that you read it and liked it! c: