The ending is absolutely awful for it completely retconned the main themes of the story.
For starters, we've been told throughout the whole series that only one side would be able to survive via completely wiping out the other; in fact that's what we've seen with the nuking of Paradis, but the way this was executed was in the worst possible manner by: (1) Completely destroying Eren's character, (2) Wasting the potential of Ymir's Character and (3) with a huge plot armor to the Alliance.
1- Completely Destroying Eren's Character: Since Eren is the main character, It would be obvious to assume that, by accompaining his journey in the first hand, we should empathize with him. We see him losing his family and friends to multiple wars, we see him getting to the basement and finding out the truth about the world, we see his worldview and reaction toward the turn of events change as his character progresses. But the best thing there was in the pre-139 Eren is that, despite all this, as we have seen in chapter 130 and 131 of the manga (I don't remember the Episodes in the anime), when Mikasa and Armin admit to themselves that *Eren was always like that*, which means that despite all the troubles and hardships his been through he was able to maintain himself true to his true self, true to the principles and the goals he previously established to himself; which is killing all his enemies (then the titans, the marleyans after and in the end the rest of the world) and his incessant drive towards freedom. To me, this always meant the true meaning why Eren inherited the Attack Titan, the Titan who always refused the orders of the royal family and always pushed towards freedom, and Eren, by being born of the seed of the Attack Titan's Shifter, was therefore the living encarnation of this will. But what does isayama do in the end? "Eren was only acting! He did it all for his friends! including killing his own mother to save Bertholdt! he was a slave to freedom all along!". Even though we've been presented with amazing plot twists by Isayama, there absolutely hasn't been a worst one than this in the history of fiction. And don't say that "That's how you wanted the story to end, not as how it has to be", that's complete bullshit. The ending simply betrays Eren's character by creating a crybaby and contradictory twisted version of himself with the narrative guise of "I was confused! I couldn't differ between past, present and future!". Don't make me laugh, that's just bad writing and It took a lot of effort to ruin such a complex and wonderful character such as Eren. This is the main reason why this ending is absolute garbage.
2- Wasting the Potential of Ymir's Character: When Ymir's Backstory was finally presented, it was great. Her personality only orbited around the fact that she was a faithful slave to King Fritz, but the her very act of compassion by freeing the pigs was the event that almost caused her death, but instead gave her godly-like powers. We see here, though latent and rather small, that Ymir also has a drive towards freedom, that probably has been manifested in the Attack Titan's will to always disobey the royals. Despite this, she remains a slave to King Fritz during her lifetime and beyond, a cycle of slavery that could only have been broken by Eren in chapter 122 of the manga. But in the ending we see this be completely rectonned. Attack on Titan, which by no means was a damned romance story, suddenly adds that YMIR ACTUALLY LOVED KING FRITZ AND WAS WAITING FOR MIKASA (Not Eren, lol), NOT EVEN A FULL-BLOODED ELDIAN, TO KILL THE GUY WHO WENT TO THE PATHS TO SAVE HER IN ORDER FOR HER TO GET RID OF FRITZ AND END THE POWER OF THE TITANS. Lmao, that's absolutely ridiculous. It came out of nothing, It looks like it was an improvised solution that Isayama came to when he didn't have enough time to finish the story. Mikasa's love for eren was - being generous here - a side and minor theme of the story and suddenly making it the main thread in order to solve some rather cosmological matter is rather dumb and confusing imo.
3- Huge plot armor to the Alliance: This topic here quite explains itself. Literally *none* of the main Alliance members died, only Magath and Hange. They literally fought a god-like being with full access to the manipulation of the laws of time and biology and won almost without effort. The whole fight was mid compared to previous Seasons and WTF was that with Grisha, Kruger and Xaver suddenly helping them even though they were under Ymir's Control? In a plotline where Eren wouldn't want to deliberately lose to his friends (which links, returning to point 1, to the complete distortion of Eren's motives) it would literally be impossible for the alliance to win, hence the plot armor.
Attack on Titan used to be a wonderful story, but the ending was way too terrible to still render It such status. It betrayed the main themes and characters, did not make sense within the same frame of the universe the story is settled and wrapped all of it with bad writing. I gave it the 1/10 grade because there wasn't the 0/10 one.
