Who in their right mind would reject an offer of unlimited knowledge and worldly pleasures?
Presumably if Faust refuses Mephistopheles’ bargain, he must resign himself to a life haunted by unfulfilled longing, existential frustration, and the bitter realization that some mysteries will forever remain beyond his grasp. Or worse, his life could descend into base forms of evil and criminality, which seems likely given what he did to Gretchen.