let's talk about the ending

Welcome to the club.

The reason why it finished like this is because they needed some conflict so that Naruse doesn't just do her play and the bitch who wrote the script https://myanimelist.net/people/5978/Mari_Okada keeps doing stuff like this where best girl gets rejected probably because she thinks it makes the story "better" or "more mature". Maybe she wanted the moral of the story to be that we don't always get what we want but we have to move on with our lives regardless.
Or maybe she just loves getting the audience invested, then bending them over and fucking them in the ass.

In any case don't even bother trying to rationalize this from an in movie perspective since there is absolutely no point. The real world people who made it just needed another conflict for the story and thought this ending would be better/more interesting/less predictable. That's really all there is to it I'm afraid.
But you're definitely not alone in your frustrations since most of the people I've talked to who've seen it seem to agree.

Yea so that ending…

So the question of the authenticity of Daiki’s feelings. We never get his pov or even a snippet of what he’s thinking, say his thoughts and feelings are closed up in his own egg if you will. Yet if we truly paid attention to Daiki and his actions we can see that he’s actually done a lot for the sake of Jun but I think the many wouldn’t really recognise that while the dynamic between Takumi and Jun seems more prominent within the film, but that doesn’t mean that they are better suited to each other romantically. I think that Daiki’s character was pretty solid and I’m happy that at the end he was able to confess his true feelings as opposed to feeling obligated to end up with the cheerleader as he’s the ‘ace,’ which even he himself seemed kind of hesitant about. I love that Daiki sees Jun and develop and falls in love with her as she breaks free from her own shell. We love to see someone going against the basic anime formula.
and plus we’re actually seeing emotional depth to a jock character??? so I’m all for it tbh.

Then there’s Takumi
Sure sure lots of us woulda loved to see him and Jun end up together and maybe I kinda did aswell for a little while, but to be honest after giving it some thought I think him and Nitou were better suited. Takumi saw himself in Jun, he saw what he had gone through, struggling to express one’s thoughts and feelings with words.
That’s why he wanted to help Jun, not some cliché i love you and I’ve always admired and adored you kind of motive, he respects how hard she is trying and helps give her a medium to express herself when she can’t find the words to.
Now Takumi and Nitou
^ they were hinted at throughout the movie and tbh I’m not surprised with the ending
It would be unfair to throw takumi into something with Jun when there’s clearly still something between Nitou and him, he clearly admits that he missed talking to her after things with them went down.
They genuinely needed their chance at their relationship seeing as their chance was cut short in junior high, they needed their chance with their unresolved feelings and clearly just unfinished business.

Now this isn’t to say that I don’t think Jun and Takumi aren’t compatible at all, but honestly I think their relationship is much deeper on an emotional level with their somewhat shared experiences and trauma, forcing them into a relationship might twist the narrative and end up making the motive of Takumi’s actions seem a lot different and build a relationship on a so called romantic connection which in reality it was something deeper than that.

AND YK WHAT
The movie isn’t even centred around romance, it’s more about friendship and communication.
So having a strong and meaningful friendship between the two main characters is perfect in my eyes.

Some of you people are way into the cliche trope stories of main female protagonist ending up with main male protagonist. This ending feels far more fitting for a high school kinda thing. Well, mostly. I don't buy the baseball kid falling for Jun after only a month like that, then again, well, who knows. Maybe cause she stood up for him too in the restaurant and other things it grew and relationships in high school are often quick to fire up and burn out. Still, I don't think that part is great, but it's not horrible.

Only thing I wanted at the end was a final scene between the mother and Jun.

Welcome to the club.

The reason why it finished like this is because they needed some conflict so that Naruse doesn't just do her play and the bitch who wrote the script https://myanimelist.net/people/5978/Mari_Okada keeps doing stuff like this where best girl gets rejected probably because she thinks it makes the story "better" or "more mature". Maybe she wanted the moral of the story to be that we don't always get what we want but we have to move on with our lives regardless.
Or maybe she just loves getting the audience invested, then bending them over and fucking them in the ass.

In any case don't even bother trying to rationalize this from an in movie perspective since there is absolutely no point. The real world people who made it just needed another conflict for the story and thought this ending would be better/more interesting/less predictable. That's really all there is to it I'm afraid.
But you're definitely not alone in your frustrations since most of the people I've talked to who've seen it seem to agree.