When Klal Yisrael said "We will do and we will hear", they were referring to the written Torah and not the oral Torah. G-d created us with an intellect, a mind, so as not to follow instructions blindy. "We will do" means that G-d expects Klal Yisrael to uphold the Torah even without understanding it, and "we will hear" refers to the open miracles that they believe in. Klal Yisrael is the only nation that can correct Adam's sin. To destroy Klal Yisrael is to destroy the entire creation.