Yeah, it’s silly to blame Ava. Maya is no stronger than Lilith, probably weaker even since she admits that she can’t power herself up with Eridium the same way Lilith can, she would not have won that 2v1 fight even without the hostage situation with Ava, her powers were getting drained regardless. And since the Calypsos likely would have learned from their mistake of leaving Lilith for their followers to finish off, they probably would have drained Maya’s life fully instead of just draining her powers and leaving her for dead.
I feel like the death was necessary, it was an important character development moment for Ava, to Ava at that point in the game the whole vault hunting thing was a game and she thought she was invincible, a typical stupid kid, seeing Maya die though taught her that vault hunting isn’t just some game. It was also important for setting up future playable sirens. Remember that there can only be 6 Sirens in existence at any one time. Gearbox likes to have a playable siren in every numbered game, therefore they have to kill off sirens at some point to make room for more. At the end of Borderlands 3 there are 3 living known sirens, Ava (who will probably be the playable siren in Borderlands 4, though she might be an NPC instead), Amara (who we will have already played as, making her an NPC in future games if she shows up at all), and Tannis. So, assuming Ava is playable in 4, we have playable Sirens now for the next 4 numbered games, assuming Gearbox plans to keep the series going that long, which they probably will since it’s their most successful franchise by a country mile.
I also didn’t feel like the writing was rushed here personally. You can talk to many of the characters about Maya’s death to get dialogue about her death, it’s just optional to talk to them. Tannis talks about the possibility of an afterlife for instance if you speak to her afterwards.
My problem with that scene is that you the vault hunter walk back into the room before it plays, yet are nowhere in the cutscene. I’m supposed to believe that my Moze just stood somewhere off camera and watched that fight instead of hopping in my Iron Bear and firing some railguns at them? I would have much preferred if we got a boss fight against them that was a scripted loss before the cutscene ever plays. Would have been a good way to show just how strong they are and how weak you are at that point in the game by comparison.