Final Fantasy 7 Remake Intergrade Ending Explained: What EVERYONE Is Missing

2. Zack & The Afterlife

To bring ALL of this back to Zack and the afterlife, I have to lean on what comes from using the Assess materia on Harbinger itself, as that states the being is an "arbiter of fate, connected to all the threads of time and space that shape the planet’s fate."

The planet's fate, is what's key.

Zack always died, and - as seen at the close of Advent Children - joined Aerith at the church in the afterlife.

Note where Zack is standing at the doors of the building in Advent Children. Note the bright white light behind Zack when he enters in Intergrade, which is once again behind he and Aerith in the movie's final scene. Both Intergrade and Advent Children's shots are almost the exact same framing, letting our heroes finally be at peace together at the end of everything.

All the children crying and people being sad inside the church - they likely gather to mourn a beloved member of the community who, no matter what, had to eventually die to "shape the planet's fate".

Aerith needed to become the Lifestream to repel Meteor, no matter what. All the main events of the story still happen - Zack dying to let Cloud live, Aerith being stabbed by Sephiroth - but it always ends the same, because the planet wills it.