Except you didn't seal the Keyholes in KH2. You opened them so you could reach new worlds.No, no, no.
Every world connected somehow. It was having the Disney villains aware of the Keyblade and of the PoH and of the Keyholes that really made the difference. It made you feel like you were a part of something bigger. The Disney worlds had a PURPOSE--to seal the keyhole, to find Riku and Kairi and the King, and to stop Maleficent's gang.
One wonders why he does this, considering this endangers the worlds that way.