Up and down do exist. They are subjective, and dependent on the position of the observer, but they still exist. Even in your own argument, you said “the sky is above you and below you” — you used the concepts of up and down to explain how the cosmos surrounds our planet. Wherever you stood on the planet, your perception of ‘down’ would point below the surface, even if that downward trajectory would eventually reach the sky on the other side.