I'm willing to bet quite a lot of money that there are more PS2 owners (even on this board) with Wiis than PS3s (including myself)... user bases don't simply transfer between consoles and console successors without changing. There is no brand loyalty in most of the market, and you are severely overestimating the effects of it on the fanbase of specific games. This is especially true with a game like KH, which failed to attract much of the PS2's teenage crowd who is the primary focus of the PS3.
Although Square Enix is the developer, the others were correct in their point that most of the game IS owned by Disney, not Square. Square was paid to make a game for Disney, not the other way around. Disney and Square both show plenty of support to the Wii, so there really shouldn't be any issues there.
Graphically, KH exists on a very delicate balance between the cartoon style of Disney movies and the realism of Kingdom Hearts. Although it's certainly possible to enhance cartoonish graphics, such enhancements are much less pronounced and have little effect. I think the game was done very well on the PS2 and could be enhanced very capably with the Wii. It could be enhanced even more with the PS3, but with the style of Kingdom Hearts, I don't think that's really necessary. It's certainly not worth the sales risks you face from a developer standpoint.
