The Real Meaning of The Umbrella Academy Season 1's Ending
The big twist of The Umbrella Academy season 1's ending is that not only were the Umbrella Academy unable to stop the apocalypse, they actually made it happen. At its very root, the apocalypse was caused by Sir Reginald Hargreeves. He raised the Umbrella Academy from children to be fighters and to always think of violence and force as the best solution (note the illustrations in the hall outside their bedrooms, teaching them how to kick and punch and gouge for maximum damage). He chose to suppress Vanya's powers when he decided that they were too dangerous, and to use Allison's abilities to convince Vanya that there was nothing special about her, leading to a lifetime of feeling inadequate. He kept her separate from her siblings, taking away a support system that she desperately needed. Essentially, he made her into a ticking time bomb.
However, Sir Reginald isn't entirely to blame. The Umbrella Academy members absolutely could have stopped the apocalypse, and were given multiple opportunities to do so. They could have included Vanya in their discussions and planning, instead of leaving her out so that the only person she could turn to was Leonard Peabody (a.k.a. Harold Jenkins). When Vanya returned to the Academy after cutting Allison's throat, Luther could have shown her compassion and forgiveness instead of offering her a hug and then betraying her by squeezing her into unconsciousness. Vanya's siblings also could have let her out of the prison cell when she begged them to, instead of walking away.
Ironically, the Umbrella Academy's desperation to stop the apocalypse may actually have been what caused it to happen. We don't know exactly what happened in the original timeline, but it's clear that they similarly came together to try and fight Vanya. Both times, the fighting is what caused her to unleash the full extent of her powers, destroying the moon and causing the meteors to crash down to Earth. If the Umbrella Academy had simply allowed Allison to go into the concert hall alone to see Vanya, the apocalypse would likely have been completely averted. Before Luther and Diego attack Vanya on stage, we see her share a warm, sisterly smile with Allison in the audience, as Allison looks up at her and finally appreciates her brilliance.
The Umbrella Academy season 1's ending is an interesting subversion of the superhero genre, which is so often about characters simply trying to beat each other by being the most powerful person in the room. In this case, the one way to save the world would have been to stop fighting, since Vanya's strength is fueled by the lack of love in her life and her anger at her siblings. Harold Jenkins may have been the fuse, but Vanya's own siblings were the accelerant.
The truly clever thing about The Umbrella Academy season 1 is that while Five is searching for clues to what caused the apocalypse - like the prosthetic eyeball and The Commission's instructions to protect Harold Jenkins - the show was actually giving us the real clues all along by fleshing out each of the characters, their upbringing, and their strained relationships. The Umbrella Academy was telling us how the apocalypse happened, but not in the way you'd expect. And by the the time the audience and the characters realize the true cause... it's already too late.