I can't see why the launch of the Destiny would require cancelling Dream sailings. At the worst,
DCL might want to move the Dream to a different dock or even a different port, or adjust the port stops.
Since there will be castmembers on both ships after the launch, they won't need additional staff for the launch. A more likely possibility is that someone or some organization charted the Dream for enough money to compensate for the missed sailings. I suppose it's also possible that something needs to be done to the Dream that can't be done during the dry dock this year but is important enough to do next year, though I have no idea what that could be.