think of it this way. As long as the people on that world never forget Sora and Sora Never forgets them they can never disapear since they do exist. (In his mind)
I don't know if you ever played Chains of memories but.... *cough*....
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**Spoilers**
A girl Named Namine is used to change Sora's memories, She takes out all parts of Kairi and she substitutes herself instead. If this had continued Sora would never of been able to get out of the castle and would of forgoten probably a lot more then just Kairi. Like I said before. I haven't played in a while and I can't remember exactly all the details.