How is the ending NOT brilliant?
It's great storytelling if you ask me. It happens suddenly, but it's foreshadowed from the beginning, it dovetails beautifully with little things about Griffith that are easily missed the first time around, and it's SO bleak and tragic. Perhaps the show was meant to be continued, but it wasn't, so you have to look at the anime how it stands, which is to have Guts trapped in an apocalyptic Hell on Earth, fighting an endless battle that cannot be won.
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I'm the Swan Queen, you're the one who never left the corps!