I get what you're saying and I respect that. But you also have to understand that these characters are written as human, I assume what people are mad about is that the characters, aside from Nino, that did not get chosen have seemingly given up right after Fuutarou chose one of them. Sure you can drag it out by having them keep battling to the death to win over Fuutarou's heart but then the manga would go on forever. Eventually one of them has to win, and the others will have to face heartbreak or accept the fact that they were not chosen.
Lets say if this situation happened with 5 friends in real life, it would be a dick move for the 4 that were not chosen to continue pursuing the love interest even though he stated which one he liked if they valued the friendship between the five whatsoever. If my friend and I liked the same person and that person chose my friend, I definitely would not throw a tantrum or continue trying to wedge their relationship apart so I can have them for myself. You only ever see that sort of thing in TV dramas or something.
Going back to the series, these 5 are sisters that all love each other very much as shown throughout the manga. And you can also see that at times, they do things to cheat each other to win the love of Fuutarou, and if they keep going at it, they will lose their relationship or bonds as sisters. After realizing their relationship as sisters is more important than the love of one boy who has already chosen who he wants to be with, it doesn't surprise me that they would respect that choice and give up.
Besides, people say the buildup throughout the entire series was thrown out as soon as the confession happened, but that couldn't be further from the truth. This is my first "harem" series, so unless there's some written rule that love is the most important thing and if you don't receive love, you are tonight's biggest loser, then just because they aren't receiving love from Fuutarou it does not mean their character development is thrown out the window. It is repeated many times throughout the manga that Fuutarou sparked a positive change within the quintuplets and those changes could be viewed to be as important as his love in my opinion:
Ichika got to pursue her dream of being an actress after Fuutarou told her in the fireworks festival to stop hiding her feelings behind fake smiles and to be genuine with them towards not only her sisters and everyone else but to herself as well.
Nino learned that even though growing up has caused the Quints to no longer be the same in both looks, likes, dislikes and interests, they are still a family. Like Fuutarou tells Miku to which Miku repeats to Nino "They are all 20 points to a whole" and there's nothing wrong with that. Then she is finally able to let go of the past when they were all the same and accept the fact that they all changed and realize that she herself has also changed and accept that they can still be family that way.
Miku was able to finally be proud of who she is by seeing there are things that only she is good at and know that no matter what happens, she cannot be one of her sisters and her sisters cannot be her. You can also say that she was able to express herself more.
Yotsuba learned that although she was the cause of her sisters all moving out of their previous school, she was never a burden towards them and they never viewed her that way as she made it possible for them all to meet Fuutarou
Itsuki discovered that no matter what she does, she will not be able to be or replace her Mom and she accepts that fact, she takes what she learned and felt towards Fuutarou and ultimately uses that experience strive to be a better person.
So yeah, although there are 5 choices, and in the end there will be 4 losers unless polygamy, it does not mean that everything that happened throughout the manga was for nothing. These sisters are just being human and respecting the fact that they are glad to have met Fuutarou and that they respect his decision to have chosen Yotsuba (well so far, only Itsuki has really said to the reader that she respects the decision and won't chase after him, the others can still do it)