Of course the change Christ brought to Judaism - for those who chose to follow him, anyway - is very different from the one Joseph Smith came up with, but it certainly was a paradigm shift and a switch of religions. A Messiah had been predicted, not a Son of God, nor God himself made flesh. That's a pretty huge difference, so you can't blame the Jews who weren't persuaded... and then you get all the later prophets - Muhammad, Shabtai Zvi, Joseph Smith, the Bab'ullah and Baha'ullah, and no doubt dozens of lesser-known ones - each of whom claimed to be a new prophet with a new message from God that did indeed contain new information, and created a new religion or denomination in the process as many believers refused to follow suit. I don't know too much of the details, but I remember seeing figures suggesting that Shabtai Zvi was just as successful than Jesus, or more so, in terms of the amount of Jews who were convinced that he was the Messiah - until he publicly converted to Islam, anyway, his group of followers dwindled rather fast at that point, even if the movement survives to this day.

So it's definitely a valid point to make - if you suppose God really did send a new prophet, or Himself again, it does seem highly unlikely that all Christians would believe it and follow him - so it might result in yet another religious split.