Exoteric, mainstream Christianity would be devastated if Jesus turned out to be a mythical character, because that kind of Christianity put all its eggs in the historicist basket. Christianity is supposed to be the only religion that isn’t based on an unfalsifiable myth or metaphor, but on historical reports of actual miracles. Only in Christianity is God supposed to have been not just transcendent but fully immanent as an incarnated man. So only in Christianity is God supposed to have firsthand experience of mortality, enabling him to form a relationship with us. All of that goes out the window on the Jesus myth view.