It's going to be tough to describe the masterful work of this Miyazaki film without spoiling, so here's an obligatory spoiler warning! (If you haven't watched this yet, then this "review" probably won't make a lot of sense.)
I remember the first Ghibli film I had ever watched--partially. I caught around 30 minutes in the Texas Children's Hospital waiting room, at around 10 years old, of Howl's Moving Castle. I remember being completely entranced before snapping out of it after being called on by the nurse for my checkup. There was a certain quality about what I was watching that was different than most of the typical western animated films; it was enchanting. It was right around the part where Howl's castle appears trudging through the high plains, with Sophie gawking at it's monstrous presence, before she boards the castle. That scene was the only thing I remembered of the movie clearly, until I watched it in full years later.
Now, after seeing pretty much every film in Miyazaki's filmography, we come to The Boy and the Heron. This film when it was announced was marketed solely off of Miyazaki's name; his return to animation after his seventh retirement, originally announced under the title How Do You Live? When I heard that this would be Miyazaki's rise out of retirement I was excited, but also a little disappointed. His previous film, The Wind Rises, was--in my opinion--a perfect end to his career. I had watched it multiple times and felt that it was a fitting conclusion to an incredible career: a self-examination of Miyazaki himself told through the story of chief engineer of the Mitsubishi A6M Zero fighter aircraft, Jiro Horikoshi, during WWII.
The message I drew from The Wind Rises was a self-reflective one. Despite how whimsical and childlike Miyazaki's worlds were, he was infamous for being a nihilist, and sort of an asshole. He would yell at animators for not recognizing his vision, he was dismissive of his family members when making his films (there is a well-known story of him being cold towards his son for not being that good at animation), and he held a hatred towards humanity. The message in The Wind Rises is consistent with his worldview: no matter how pure his intentions were when making his films, he always had to sacrifice something in order to make them. Jiro's fighter aircrafts acts as an allegory for Miyazaki's films.
However, if The Wind Rises acts as a reflection on Miyazaki's legacy, then The Boy and the Heron acts as a hopeful look into the future. To frame these two films within the context of his legacy, if The Wind Rises is Miyazaki saying, "What have I done?" then The Boy and the Heron is him saying, "I've done all I can."
There's a particular moment near the end of the film where the grand uncle gives Mahito the opportunity to take over the tower, to become its pure-hearted maintainer. It's made clear throughout the film that the grand uncle is attached to this world he's created, pretty much to the point of obsession. However, when Mahito refuses his offer, the grand uncle doesn't react negatively. In fact, how I saw it, the grand uncle became relieved. No longer would this broken world be upheld by the hands of a singular person; it shall crumble. And that's the beauty of the film. Despite how nihilistic Miyazaki is--at least the nihilism he shows to media outlets and camera crews--there is an optimism that acts as the driving force and ultimate conclusion of the film. Even if it all comes crashing down, it can always be built somewhere else, by someone new. My interpretation (and many others) is that Miyazaki is both Mahito and the grand uncle. The grand uncle represents Miyazaki's obsession. Decades dedicated to the project that is Studio Ghibli, a legacy that he would like to continue through his son. Mahito represents Miyazaki's acceptance that such a legacy is impossible. That no matter how pure-hearted his intentions were, he was not able to change humanity for the better--essentially, he sacrificed so much for nothing. But that's okay. Because you can just build a new tower.
The ending, no matter how abrupt some people thought it was, left me speechless. For the first time, I saw a Miyazaki film where he spelled out, in plain animation, exactly what he wanted to. It can be considered a cliche, but ultimately its true: change happens, and it's okay that it happens.
Rating: Miyazaki don't go please