It has been three years since Steven Universe came to a close and there's still a lot of discourse over the ending. Steven Universe (or SU for short) was always a show more about themes and ideas than the actual plot. While the entire arc of the show was planned out, writing was never the focus of the show. It was ideas, authenticity, and animation that took the focus.
The episodes were written as a collaboration between the storyboard artists and story editors rather than in a writers' room. While this helped maintain SU's calm aesthetic and environmental storytelling, it left a lot of character arcs feel somewhat lacking. Things would be set up in a way that was really engaging, but then the following episodes would just be slice-of-life episodes that don't advance the plot. Then by the time the episodes dropped that actually did move the series plot forward, it felt anticlimactic. But even with all off that context, the ending of SU still manages to perfectly and succinctly sum up everything the show stood for.
What didn't help this was that because of the show's inconsistent release schedule, fans would go months, or even up to a year, without any forward momentum in the show. However, this issue has been remedied now that it's streaming. Most episodes only run about 10 minutes long, making it perfect for binging. Without the long wait between episodes, the story arcs feel much better paced even though there is an asinine amount of filler.
But the biggest problem people seem to have with the finale is with how the main conflict resolves. In the final arc of SU, Steven and the Crystal Gems make it to Homeworld to confront White Diamond, the person who has been running the Gem empire this whole time. White Diamond and the other Diamonds have been responsible for the subjugation and destruction of entire planets for thousands of years. Being responsible for so many atrocities and Steven still manages to change their minds and convince them to do good rubbed a lot of fans the wrong way. But this ending is also the perfect summation of the central theme in Steven Universe: to be empathetic because life is more complicated than you think.
Because the situation is more complicated than most cartoons, the politics and social structures of Homeworld have been explored in depth throughout the series, so it's been well established that just getting rid of the Diamonds won't solve the generations of tyranny. This was always going to take time to fix. Steven didn't forgive the Diamonds. He recognized that they can help undo their damage because they're still some of the most powerful beings in their universe. They can literally use their combined powers to heal all the Gems they've hurt or killed. The issue is that the Diamonds won't budge on their stances.
White Diamond's power includes the ability to literally take over the minds of her opponents and make them extensions of herself, like The Borg in Star Trek. At this point, the main subtext of the show being a metaphor for family dynamics becomes literal text. White Diamond is Steven's grandmother, she is the one who instigated the generations of family trauma and caused their world to be broken. And the solution in this scenario? Steven risks his very being to show that his existence as yet another generation in the family means that there's still chance to heal.
No one is ready to accept The Diamonds fully back into their lives yet, but Steven succeeded, and they used their powers to heal the damage they've done. While things are repaired, Steven and the Gems make it clear they still need distance and that they're going to stay on Earth. They are appreciative that the Diamonds worked so quickly to begin repairing, but they are a long way from giving them any forgiveness. It's a start, and the broken family can become whole again.
Steven didn't kill the Diamonds because this show was never meant to be that cut and dried. The Diamonds needed to take responsibilities for their actions, not because of bad faith story criticisms, but because this show was always about breaking the chain of abuse. And the first step to that is always acknowledging there's a problem. Years of damage cannot be wrapped up in a neat little bow. This was never a show about death; it was a show about continuing to live.
These ideas are even reinforced in the show's epilogue TV movie and limited series, Steven Universe Future. In those shows, the relationships between the Diamonds and Gems has completely changed. They relinquished their powers to the Gems of Homeworld and helped reform their social and government structure. The Gems who remain on Earth are still wary of them, and with good reason. But the Diamonds still help anyway when Steven is in danger.
The focus of the finale's criticisms tend to be centered around these issues from the Diamonds. It was viewed as being rushed, unearned, and lazy. But when taking a step back and looking at the series as a whole it fits so perfectly. It shows that even though the chain has been broken, there's still plenty of work to be done and life will continue to go on. The follow-ups to SU have even explored how all of this has psychologically affected Steven.
While the relationships are seemingly improving on the surface the long term damage is still causing him struggles and the trauma effects his mental state. This isn't even made subtext, there's an episode where Steven is getting tests done at a hospital to figure out what is wrong with him mentally and physically. Confronting these issues makes him have a panic attack. The chain has been broken, but that doesn't mean it's fixed.
The messaging is abundantly clear. While things are broken, the only way to make things better is to make a change.
This message is reflected in the lyrics of the song Steven writes at the end of the finale:
"I don't need you to respect me, I respect me.
I don't need you to love me, I love me.
But I want you to know that you can know me,
if you change your mind."