Three things -
1) it doesn't take into account that Gaia made the Dragon-Blooded to be the Celestials' foot soldiers the Primordials didn't curse the Exalted
2) they tried cursing the Incarnae, but they were able to protect themselves from the curse, so the curse bounced onto the Exalted
3) the Celestial shards are reincarnated into new people because the power is too great to pass through the blood and because the Incarnae didn't want a dynasty to usurp them the way they usurped the Primordials
but those are little details...
1) it doesn't take into account that Gaia made the Dragon-Blooded to be the Celestials' foot soldiers the Primordials didn't curse the Exalted
2) they tried cursing the Incarnae, but they were able to protect themselves from the curse, so the curse bounced onto the Exalted
3) the Celestial shards are reincarnated into new people because the power is too great to pass through the blood and because the Incarnae didn't want a dynasty to usurp them the way they usurped the Primordials
but those are little details...