I know everyone's probably getting tired of time travel questions, but what the hell. We know that Xehanort planned to gather thirteen vessels from across time to complete the X-blade. And in he did so in the end of DDD, but since Sora was rescued all of the "Xehanorts" went back to their original places in the timeline. Xehanort then said that his thirteen darknesses would clash with the seven lights sometime in the future. This is what we know.
Now this if where it gets a little confusing so I'll try to explain it the best I can. Since the other Xehanorts already exist in their own time periods, wouldn't that mean that Xehanort has already failed? If Xehanort had completed the X-blade in the future, then his thirteen selves wouldn't be able to go back to their respective times. The only way I can see this working is if completing the X-blade somehow messes with the timeline.
Now this if where it gets a little confusing so I'll try to explain it the best I can. Since the other Xehanorts already exist in their own time periods, wouldn't that mean that Xehanort has already failed? If Xehanort had completed the X-blade in the future, then his thirteen selves wouldn't be able to go back to their respective times. The only way I can see this working is if completing the X-blade somehow messes with the timeline.