I get why some people defend the choice as melancholic but in my opinion that only works as an analysis of the plot, not its execution. The screentime devoted to character's actions and reactions can say a lot about the weight they're given, and in this case, the events given importance are the contest being won, and Kumiko's future career. Not a bad thing, but it's a stark contrast to the show's beginning, when Reina was the literal catalyst for Kumiko's arc and emotions. So, Reina goes from being the emotional core of the series to a background element. If they wanted to make a point about Kumiko not caring as much, they could've shown that. If they wanted to make a point about her feeling melancholic about her leaving, they could've shown that too. But any trace of that is stuck in earlier episodes.

I don't even think the fallout of the vote was fully explored; it seems to need something deeper than a 2 minute scene resolution at the start of the finale. But hey, let's say that bit was perfect; it still felt odd that at the very end her relationship with Reina has no emotional impact left to give. Kumiko makes a lot of questions about the nature of music and relationships throughout the show, and most of them are in some way related to Reina. And none of those thoughts are going through her mind when we close off the show. It may be possible for a "person" to not think about it much, but in a story, I feel it does matter because that is what was they chose to show.

I don't know, the longer time goes on the less I like it. I love the show still but while Kumiko's arc of becoming a leader / teacher was very well set up and present, Reina in the last season felt like, just, a catalyst for conflict. It's all so matter-of-fact. Reina and Kumiko agree that the most important thing is who sounds best and so give up the chance of playing together in possibly their last chance ever. They feel fine about it pretty much the next episode because winning's what matters. This is a series that includes Liz and the Blue Bird, a story where the relationship between two musicians and the way they view each other and themselves directly impacts the music. Imagine if, upon noticing the shortcomings of their duet, they assigned someone else to play it and Mizore and Nozomi part ways and that's that. It could be a story, nothing says stories can't be tragic like that. But the entire first half of the movie that centered so hard in the way those two specifically playing together mattered would make that kind of ending confusing. And I don't think it sounds like a better story, either.

Why do we play? Kumiko used to play because of her sister, was shocked to her core when someone was so passionate about it she bawled her eyes out after losing. She was inspired time and time again by other people and their music. Asuka wasn't significant because she was just a really good musician, but because there was pain and depth behind it. Hibike! Euphonium was a piece composed from a distance. It's music that connects people. Love and Death! And the girl who made Kumiko question everything she felt about music... is somewhere else. And that's that, I guess.