For as the angels were mismanaging the world, owing to love of power, he (Simon) had come to set things straight, and had descended under a changed form, likening himself to the Principalities and Powers through whom he passed, so that among men he appeared as a man, though he was not a man, and was thought to have suffered in Judaea, though he had not suffered. –
(Refutation of All Heresies, 6, 19).
I mean: when the Magus did that claim (identity with Jesus), he was assuming implicitly a distinction between how Jesus was conceived by others - viz. as a historical man - and how he had to be conceived by a Simonian: as Simon himself, really.
So if Roger Parvus is right to consider the Ascension of Isaiah and a lot of pauline portions as later Gnostic fabrications designed to make a already existing image of Jesus as the distorted and judaized image of the real "Jesus" (Simon Magus) , then these Gnostic efforts (of co-optation) prove logically that that "already existing image of Jesus" placed him in a precise point of the History in Judea.
They weren't inventing that previous Jesus, they were co-opting him. So he had to exist in first place, to be co-opted.
How may the mythicists reply against this serious argument? I don't think that the negation of the historicity of Simon Magus may resolve the problem. They may reply by pointing out the fact that Jesus was already euhemerized by "Mark" well before Simon Magus was co-opting and usurping his human legacy.
So the mythicists are moved to consider Simon Magus or the his similar "co-opters" as the figure of the "false Christs" meant in Mark 13. But then Jesus was already euhemerized before Mark 13 was written. Then Mark 13 was probably a later addition to proto-Mark.
So, while it seems to me that Jesus never existed, I think that this evidence about Simon Magus is the best evidence, until now, of the historicity of the man Jesus.