Marvel's Uatu the Watcher is a fan-favorite character, but despite his power, he rarely intervenes in the affairs of "lesser beings" even as they face the world-ending might of Galactus. The Watcher had made a well-known solemn vow never to interfere, but that hasn't stopped him from doing so on dozens, if not hundreds, of occasions. However in What If...? #41, Marvel reveals that the Watcher's power actually exceeds Galactus' and Uatu could have stopped him whenever he wanted, though it would require great sacrifice.
Both Uatu and Galactus debuted in the Fantastic Four's Galactus Trilogy. Created by Stan Lee and Jack Kirby, the three-part series involved the Fantastic Four's efforts to stop the mighty Galactus from siphoning the Earth's energies, essentially eating the planet. Galactus was a rare breed of villain at the time; he didn't act out of malice or hatred towards humanity, but rather attacked out of an instinctual need to consume planets (he saw neither good nor evil in the act, but necessity). Because of both the villain, heroes, and the all-seeing Watcher giving out advice whenever possible, the trilogy is seen as a classic.
In What If...? #41, the story plays out once more, only this time the Fantastic Four are nowhere to be found, and the Avengers take their place in the story. This small change leads to greater consequences, such as both the Silver Surfer and the Watcher deciding to intervene (though not before the Watcher and Thor fight). After the Avengers fail to take the FF's more considered role in stopping his attack, Uatu decides to directly intervene. Galactus kills the Watcher - but this act satiates his hunger and his attack on Earth immediately ceases.
Galactus is stunned; he has never seen a Watcher intervene in terrestrial affairs before, much less battled one. For the first time in his history, Galactus shows true remorse and takes the Watcher's body to his home planet. While the moment is tragic, it proves that the Watcher could have saved the Earth at any given moment with the ultimate sacrifice. It's unknown if killing the Watcher satiated Galactus' hunger temporarily or permanently, but the planet is saved regardless.
Galactus, like the Watchers, has existed since near the start of creation, destroying countless civilizations in his unending hunger. In this story, it turns out that the Watchers have always been able to satiate his hunger to the same degree - a debt they arguably owe the galaxy, as Reckoning War reveals their early efforts to influence sentient life created an unparalleled threat and left the majority of the galaxy uninhabitable. While the Watchers are incredibly powerful and sworn to non-interference, it turns out they've been valuing their own lives more than endless worlds consumed by Galactus.
In the What If...? series (much like the 2021 Disney+ television show Marvel's What If...?), the true Watcher narrates the story as it progresses. This time, at the story's conclusion, he admits it is hard to watch; "It is so difficult for me to observe. For in this reality... I died." This means that Earth-616's Watcher is fully aware he could sacrifice himself to save at least one planet, and even to potentially end Galactus' hunger, but has repeatedly chosen not to. As kindly as the Watcher has seemed in the past, it turns out he has allowed Galactus' rampage to continue unabated, despite the very real option of having the near-omnipotent race pay their debt to the galaxy by sacrificing themselves to sate his hunger.