Short version: claims that the Christ myth has been based on older deities have been around for a while, but they never stand up to scrutiny. Clearly the full Christian mythology as it developed owes some thanks to various other mythologies, including Greek and Zoroastrian. But the idea that there are a bunch of other gods that match up with Jesus on all these specific details is just hogwash, based on conjecture and misinterpretation. The history of myth simply isn't that neat and clean. Certain aspects of the Christ myth can be easily explained by reference to the actual situation of early Christians, without need to make conjectures about gods that might have existed, and without appeal to gods we know existed, but for whom we don't have evidence regarding the specific details. For example: it's true that death and rebirth is a common motif in many mythologies. It's probably a mythical universal. But the story of Jesus' resurrection was the direct result of a very specific event: he got killed, and his followers were not expecting it. The resurrection was how they explained his death. We don't need to suppose that maybe it had something to do with Mithra or Horus or whatever. The idea that a divine being could rise from the dead was readily accepted by everyone. They didn't need specific gods to prove it. It was common knowledge it could happen, so they decided that must be what happened to Jesus, because his death simply didn't fit in their worldview.
EDIT: Please note the fine line I'm drawing. I'm not saying the Christ myth was unique in any way. I'm just saying the idea that the Christians picked and chose from specific other gods is unfounded. They were just using the various explanations available to them at the time.
