Even earlier than that. Sword Art Online turned bad after episode and steadily went down throughout the SAO arc. Come ALO arc, the quality of the writing took an Olympic nosedive into a dumpster.
Many fanboys consider SAO to be original. In reality, it is a rip-off of Hack Sign. But the main problem with the anime was that it lost it’s original focus. The original focus of the anime was trying to be an anime about clearing the game, which then turned into an anime around unnecessary side stories, CSI, fishing, and much more unnecessary side stories. In fact, only about four episodes were actually relevant to the SAO arc. The time skips miss important points in plot and characters development. Then once the ALO arc occured, it took a nosedive into a garbage dump. In SAO, when you died in the game, you died for real. The story in SAO was about clearing the game. In ALO, the story was about Kirito becoming a fairy and trying to save his waifu from a rapist. In ALO, you cannot die, so there was no sense of suspense. Plus the show lacked urgency. At one moment, Kirito will be trying to find Asuna, only for him to derp around with some useless alliane or with his Imouto. Any conflict that does happen is only solved by a deus ex machina. Episode 24 had literally, the biggest deus ex machina, out of any anime I have ever seen, and that is not a good thing. Plus, ALO devolved further into weird and disturbing Otaku fetishes like incest and tentacle rape .
The animation isn’t great either. While the backgrounds look nice, when the story has to be animated, character expressions look distorted (Kirito vs Eugene is a good example of this) and acton scenes look like gifs of moving light. The music is alright. Yuki Kaijura composes great soundtracks for anime, but compared to some of her other works, this soundtrack was quite weak. The only notable song is the coffee-soda song, which is played in almost every action scene, decreasing its impact as the story” goes on.
One of the key aspects of SAO was the romance between Kirito and Asuna. The romance was incredibly forced and felt like it was written by a poorly written fanficiction due to Kirito and Asuna having no chemistry. It came out of nowhere with little to no development. How am I supposed to take two teenagers being married, having an A.I. daughter, and stating that they will be together forever seriously? After ALO, the romance turned into a love revolving around a 16 year old gamer (Kirito), a comatose girl (Asuna), and his cousin (Suguha). The characters SAO focuses on the main character named Kirito. In literature, there is a term that refers to perfect characters as gary stus. Kirito is the epitome of a gary stu. He’s perfect, he gets all the girls, he gets the only special skills, he gets special gear that nobody else gets, he breaks the game’s rules, and he is the moral paragon. He is only popular among the fans because he is easy for the fanbase to insert themselves into Kirito’s shoes. Asuna, the main heroine, starts off as a mysterious and independent character, who latered turned into a generic tsundere who cannot do anything without her boyfriend. In fact, all the female character cannot do anything without their boyfriend Kirito. The only other significant male leads, that are not villains, are either wimps, or are only given about one or two episodes of screen time. The reason for that is becuase if they recieve any more screen time, then that will taint Kirito’s awesomeness.
Then there are the villains in the story. The villain in the SAO is Kayaba Akihiko, who traps 10,000 players for over two years inside Sword Art Online. At the end of episode 14, Kirito asks Kayaba why he trapped all these people, killed over thousands of players. His response was that he did not know. If you are going to write a villain, you better give him a good reason why he would do all the things he has done. So Kayaba is a poorly written villain, but he has the complexity of Johan Liebert compared to ALO’s villain, Sugou. Sugou is the kind of person that you find in a hentai, a rapist who is evil for no reason, only to make Kirito look like a better person. A middle aged man who wants to rape an underaged girl yet he screams like an under aged girl. The character consistency is also bad. At one moment, Kirito will go Guts from Berserk and brutally kill whatever he sees and then the next moment, he will act like a friend, for no apparent reason. Kirito is claimed to be an antisocial loner, but he has no problem talking to others and adding girls into harem as if they were pokemon.