Originally Posted by
Val the Moofia Boss
Anime adaptations rarely ever finish the story.
Anime adaptations are usually commissioned for the purpose of advertising the source material (manga, web novel, or light novel) to a wider audience. These adaptations are usually commissioned when the popularity of the source material is on the rise. When the popularity of the source material declines, it is very rare for it to receive further anime adaptations. Thus, why you have dozens and dozens of anime adaptations that do not finish the story.
Full Metal Panic (a light novel series) was insanely popular during the 2000's and received three seasons. Then it was forgotten. It's a miracle that it received a fourth season all of these years later, but it is unlikely that we will ever see the story finished onscreen.
Re:Zero was the most popular anime adaptation of a light novel I've seen in years, but I am doubtful that the whole story will be adapted into anime. The license holder didn't immediately jump on the chance for a second season, instead waiting years to announce it. The hype for the series has significantly died down. The arcs also get longer and longer as the series goes on, to the point where a 24 episode season might not be enough to cover an entire arc without cutting content. I doubt we will see the entire story adapted into anime.
There is also a special category of anime adaptation: Shounen Jump adaptations. The manga publisher, Shounen Jump, prefers to milk their properties for as long as possible. You may remember that they commissioned anime adaptations for their big three manga series, One Piece, Bleach, and Naruto, and ordered the series to go on indefinitely. Thus, why their anime adaptations are riddled with record breaking amounts of filler and padding. You need to watch Naruto Kai, a fan recut, in order to consume the anime adaptation of the story in a timely manner, whereas it is virtually impossible to do so with One Piece as even non-filler episodes are laden with filler scenes. The Naruto manga was headed to naturally conclude with the defeat of Pain and Madara, but Shounen Jump couldn't make money off of a concluded story, which is why Kaguya was introduced. The Bleach storyline was prematurely axed when its popularity had waned, which is why it had a rushed conclusion. Black Clover is the only other Shounen Jump adaptation besides One Piece that is slated to go on for forever with fillers. It's a miracle that Assassination Classroom and My Hero Academia didn't get slapped with anime adaptations meant to go on for forever, which is also why their anime adaptations are so good.
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If you are interested in Naruto, there is a fan recut of the anime called "Naruto Kai" that tells the entire story in just 72 episodes, only 8 episodes longer than Fullmetal Alchemist: Brotherhood.