When I read Muslim apologists, I'm struck by how shamelessly they quote "skeptics" about the Bible and church history. In that vein, it's useful to compare skepticism about the existence of Jesus or NT history and Christology to skepticism about the existence of Muhammad.
You have mythicists like Robert Price and Richard Carrier. For his part, Hector Avalos clams to be agnostic about Christ's existence. This, in turn, has parallels with the minimalist school of Biblical archeology.
Now, compare this to scholars who deny the existence of Muhammad and the traditional historical narrative.
My point is not to endorse that position. But if you deny the existence of Abraham or Moses or Jesus, if you deny the Exodus, why not deny the existence of Muhammad? Be consistent in your methods and assumptions. Radical skepticism about Jesus and the Bible doesn't come to a screeching halt when we change the subject to Muhammad and the Koran. The same logic (or illogic) applies with equal force.