There is no need to think particularly “deeply” here. This is literally the embodiment of “they will all be better off without me” and other suicidal experiences when a person feels like a problem that prevents others from living. In terms of storytelling and developments, I think just because there are topics like these in the industry doesn't mean they'll be relevant. It's like saying that the ending where Freeren is killed by a random arrow during a fight is completely appropriate, because in the Middle Ages you constantly saw people killed at the wrong moment by a random arrow.
Oshi no Ko, on the contrary, strived, seemingly outwardly, to be and be perceived by people as an inspiring story about second chances and overcoming the dark aspects of show business. Especially considering how the characters developed and the problems they faced. An ending in the style of Fincher or Tarantino not only cynically destroys this and drives people into depression instead of any motivation, but also clearly greatly shifts the emphasis and vibe of this work. You can even test this for yourself by trying to read old arcs after this ending.
I mean even Tarantino's most acclaimed movies had satisfying endings (Django, Inglorious). It made you feel good that the heroes triumph.
As you say, the previous arcs feel pointless and hollow, because A) these characters' journeys all feel wasted. and B) we barely get to see how all the other characters move on, its one funeral and then time skip where Ruby just becomes Ai (the point established that she was supposed to avoid).
The tone took a heel turn once Aka started incest-baiting us for NO reason because it never went anywhere. I guess he wasn't mature, patient or interested enough to tackle that subject of the relationship dynamic between Sarina/Ruby and Goro/Aqua.
The tone took such a heel turn once MORE when Nino (whom we never met before) is introduced as the "final big bad", but then WAIT, Hikaru was the big bad... again, all along! This was extremely sloppy. Akasaka "planned" for Aqua to die from the beginning, new developments be damned.
IMO this is Akasaka vindictively ending a story bc he was either bored with the concept or bitter at the fact that Love Agency got axed.