I mean, I have never played any KH game yet, but I gather it's a series that requires you to have played previous games in order to get into its complex storylines twists, so this could put off whoever would feel like jumping into the series for the first time.
Besides, IIRC KH games were million sellers on PS2, but didn't reach one million mark neither on DS nor PSP, so if KH3DS should sell more than any handheld iterations, it would be something big in my understanding.
Of course KH3D will sell decently in the worst possible scenario, but I am not sure whether it's going to underperform or sell more than a million, perhaps having long legs, unlike SM3DL, MK7 and MH3G that were pretty much safe predictions as far as I'm concerned.