I share your opinion on many points, her death gives us the feeling that she was only a tool for Ruby and Jaune's development. It was completely avoidable, as you said. Beacon and Vale were already doomed, ever her death couldn't have changed it, no matter how hard she would have tried. The intention behind her sacrifice is quite hard to get (if we don't consider the silver eyes thing, that even she could not have predicted)...

Ozpin is to blame for what happened too. He put such pressure on her because of the Maiden Power, as she had to chose between everything she was and loved and the future of Remnant... Qrow said in volume 4 they gave her a choice, but she never really had it. She was too kind, noble and selfless to refuse what her sense of duty was telling her to do. After that, there was the shock of being the one who accidentally killed Penny... And finally she watched Cinder steal the power she was ready to sacrifice herself for... Everything around her was falling apart before her death, and she had no control over it. I don't know what she was thinking about when she understood Cinder had won over Ozpin. But she was completely upset by everything she had been through, and maybe that's why she acted so irrationally. Of course, it was not a wise, "Pyrrha-like" decision to abandon Jaune to rush into what was basically a suicide mission. But sometimes humans feel lost and break down, and it can lead them to do things they would never do in other circumstances. However, I'm not sure if the show runners thought that it was rational to make her act irrationally. They probably just decided that it was the most shocking and heart-wrenching thing to do for both the fandom and the remaining characters...

I heard that it was Monty Oum's plan from the start to make her die ; but, just like you, I keep feeling it happened too soon. There was so much more to do with such a lovable character.