Sure, the Galactus card... you should read the coming of Galactus again. The FF were completely useless against him, Earth was saved because of the fortuitous interference of another giant alien.
And if there were no superheroes? If the security of the world was not trusted to a cabal of a few members? Good question, what can the human race even hope to do against a giant from outer space with weird strange powers? Answer: quite a lot. Never underestimate the power of human ingenuity.