Those who believe that Christ died as the only acceptable atonement for the sins of mankind wouldn't be very happy if that were revealed. It would mean their faith was in vain as the requirement was a sinless sacrifice and only Jesus (as the God/man) could measure up to that.
Yes he was a man but he was certainly something more than just that, this much is demonstrated clearly by his unrivaled effect upon the world.

If you think about it, if this theory were even remotely true, the Jews of the time, themselves desperately trying to discredit the idea of Jesus as the Messiah, would only have had to reveal the relationship to convince the ever growing Christian exodus within their own ranks that His claims of perfection ie Son of God, were logically unfounded, yet history reveals no such claims.