Author's Saving Throw: At the end of Age of Ultron, we see Miles looking at Galactus as he appears over Ultimate New York. However, when Galactus shows up in Hunger, he had just arrived. Cataclysm #1 reveals that what Miles saw was just an image of Galactus, but it did serve as a worrisome warning for the wall-crawler.
Harsher in Hindsight: Secret Wars (2015) shows that the heroes of the Ultimate Universe only delayed the inevitable. Moreover, despite what he decided here, later stories showed Ultimate!Reed is still a villain.
Some people hoped that Galactus would wipe out Ultimate Earth, citing Seasonal Rot as a reason, and hoping Marvel starts over (which would be the result, given Galactus' nature). Given the universe's Darkness-Induced Audience Apathy nature it's understandable why people would rather root for Galactus to destroy the universe than have a collection of designated heroes continue to live there.
Some people are also just happy to see Big G finally roar out of his Villain Decay, after years of serving as the cosmic punching bag for other big villains.