To be fair, Firestar was leader of Thunderclan for nearly the entire series, so I think that it was a good ending if they didn't make that one book that happened after the Firestar's death.
Let's have Firestar die even though he's the most important character; What a great idea! Excellent! Nah. Let's keep him he's important; Firestar is dead. Bramblestar is a Tigerstar clone. We need Tigerstar clones; THAT IS NOT A STUPID REASON!!! YAAAAUAH; Stupid reason boss