What ultimately disappointed me about Sakura was the fact that she was one of the four main characters of Naruto (that's Team 7, basically).
People would claim in the past that "Hinata makes a better heroine than Sakura" but these are the shippers talking. Anyone who seriously compared the two of them before The Last had their shipper goggles on full power, fighting over whether Naruto should father Hinata's babies or Sakura's babies. Other than Uzumaki Naruto, they had nothing relevant to compare with each other.
Because Sakura was a main character, and Hinata was a side character. That's all there is to it.
Hinata was a side character, so expectations for her were relatively low.
Sakura on the other hand was a MAIN character, and her character development and relevancy to the plot was supposed to be keeping up with Naruto's, Sasuke's, and Kakashi's. Or at least keeping up with Kakashi's development/relevancy because one of the major focuses of the manga was the Naruto/Sasuke rivalry.
Her development in part 1 was... okay. She started out as a noisy brat who didn't understand orphans. Then she had the scene where she was unable to keep from crying when Sasuke was presumed dead in the Waves arc. She then protected Naruto and Sasuke from Sound ninja in the Chuunin Exams. Her fight with her rival Ino in the prelims was a moment of strength for her. POAL happened. And then she goes up to Tsunade and asks for training.
At the beginning of part 2 her development/relevance skyrocketed so much, fans were going crazy. She singlehandedly saved Kankuro from death by poison, a poison that no one knew how to treat, with the skills she'd picked up during the timeskip. She teamed up with Chiyo to fight with Sasori and beat him, when in the past all she could do was just act as a meat shield with a kunai.
Expectations of Sakura were hyped up like crazy... and Sakura herself just stopped. She was just there. Or used as a device, like a side character. She failed to live up to the weighty expectations that had been placed on her. In comparison, Sasuke and Naruto's characters got loads and loads of attention and relevancy. Kakashi faded from the spotlight for a while, but bounced back when Obito's identity was revealed. But Sakura's development/relevance just came to a halt after the Gaara Rescue arc. Worse, her development/relevancy took a nosedive during the Kage Summit arc.
The Kage Summit arc was arguably one of the worst arcs of part 2 and the entire series, because while the War arc created plot hole after plot hole, the Kage Summit arc gave all four main characters negative character development. Sakura got hit by it the worst, because up until then her relevancy/development had been completely static. I'm not talking about her confession to Naruto. I'm talking about when she had the chance to kill/seriously injure Sasuke, and didn't take it.
We already know about how her spine had been ripped away from her and all she began to focus on was Sasuke, resetting any development she had left, so I won't talk about it here. I think everyone would agree that Sakura was beyond saving when Sasuke told her "sorry" and she replied with "for what?".
Sakura had the potential. Kakashi had pointed out in part 1 that Sakura was intelligent, had a talent for chakra control and genjutsu, and was "the same as Naruto" in the sense that she hated giving up and refused to go down easily. In part 2 Kakashi also noted that at the rate of her development, Sakura would soon surpass Tsunade herself. Chiyo commented that Sakura had a noble heart, which was rare (among kunoichi). Ignoring the slight sexist undertones, Chiyo was referring to Sakura's strength of character as a person and as a shinobi. Sakura had the potential... and she failed to live up to everyone's expectations.
So.
Sakura got replaced by Hinata as a main character in The Last.
Now that the focus was on Hinata and not on Sakura (who was officially a side character at this point), we see the exact same problems.
Hinata actually has a workable backstory: her father issues, her self-esteem issues, and the Hyuuga Branch enslavement issues. Mysteriously, they are never brought up, and her shyness isn't acknowledged by the narrative as a character flaw. Instead, it gets heavily romanticized.
Hinata is shown to not have changed at all since her part 1 self. She is unable to give Naruto a scarf without some serious prodding from Sakura. Actually, it's even worse because in part 1, Hinata mustered up the courage to give Naruto the medicine herself (and succeeded), but in The Last she was literally unable to talk to Naruto for two whole years, and was unable to give the scarf to Naruto until they were in the middle of dangerous enemy territory, with her own sister suffering from missing eyeballs close by.
When she learns about Toneri's plans to destroy the world and what he did to Hanabi's eyes, she keeps the information to herself instead of informing her teammates about it, then proceeds to romantically frolic with Naruto through a destroyed city and fix the red scarf. When she finds out that she is the Byakugan Princess in that very same city, destined to save the world, from Hamura Otsutsuki himself, she again keeps that vital information to herself instead of coming up with a plan together with her teammates and continues to knit the red scarf, then goes off by herself with Toneri without a word. She isn't even shown worrying about Hanabi, her own sister, at all.
Despite her newly acquired powers she'd gained from Hamura as the Byakugan Princess, which supposedly makes her stronger than both Naruto and Sasuke, she is unable to follow through and destroy the Tenseigan. Even if her success was dictated by destiny itself. Toneri, who doesn't trust Hinata at all, for some inexplicable reason covers his entire face right in full view of her because the pain of his developing doujutsu is too great, and Hinata doesn't capitalize on it. It's Sakura failing to capitalize on her chance to kill Sasuke all over again, except now it's Hinata's turn to fail.
The only important things Hinata got to do in her own movie were: 1) Infiltrate Toneri's castle 2) pluck out Hanabi's eyeballs from Toneri's eyesockets 3) Do a combination attack with Naruto.
And then when everything ended, Hinata invites Toneri, the same guy who nearly ended all life as we know it, the same guy who tried to force marriage on her (and most likely rape her later on), the same guy who stole Hanabi's eyeballs, the same guy who destroyed Naruto-kun's red scarf that she worked so hard on since the beginning of this movie, to live with them on earth. Toneri refuses the offer and decides to live alone on the moon to repent. Basically, Hinata did the same thing as Sakura and instantly forgave Toneri for all the atrocities he'd committed, and Toneri pulled a Sasuke by staying away to "repent" but instead of becoming a wanderer, he's on the moon. And thus he got away with everything. Sasuke had the excuse of being one of those who saved the world, what's Toneri's excuse?
When the Bolt movie came out, she was made irrelevant again.
The conclusion is thus: Sakura failed as a main character, and her relevancy/story arc/character development just went full circle, all the way back to her chapter 3 self. And Hinata also failed as a main character, because she also went full circle by going from being irrelevant, to being a relevant main character and failing to meet the standards placed on her, and then falling quickly back to her irrelevant status. If the title "heroine" means "which girl successfully banged Naruto" and nothing else, then I don't give a damn anymore.