If it's true Xehanort's menacing legacy will be done and done... then what will 3 be about? Frankly, I don't think Square's planning a 3.
Final Fantasy has been great because each story is it's own. There may be plot holes and unanswered questions, but most of the time, the player is left satisfied that the big bad is dead and the world is at peace. Sequels that involve an entirely new menace are usually a flop. Just look at DoC and X-2 for proof. Despite the fact that these supposed new menaces were at least in some way related to the original big bad of the game which spawned them, both were still poorly received. It's harder to build a story around an enemy with different motivations from the original. So the game ends up feeling different and not in a good way. Don't get me wrong, I loved DoC and X-2, but I've played other games' sequels and spin-offs I couldn't enjoy as much as the original so I feel I know where this seemingly misplaced spite is coming from in fans who dislike spin-offs.

Someone stated on that site that there is no way the Xehanort Saga is ending with 3D. We'll have to play through the full Kingdom Hearts 3 on PlayStation3 before Xehanort is gone for good. A true trilogy. The only other option is to make 3D the last installment in the trilogy and to follow with Kingdom Hearts 4 on PS3.