What you're saying sounds like you're talking about the people who'll buy it just because it's heavily Disney related. Your statement about how each world is based on movies are true but when I played KH, each world had pretty much just the core plot and climax, they were all really short. Also your actions were way of topic. You were fighting enemies that were from Kingdom Hearts and not those specific worlds half the time.
People who buy movie games expect much longer and much more detailed and focused experiences on that particular movie than what KH games offer. It just doesn't seem right to even compare KH to movie tie-ins when you consider this. It's more like a cross-over game.
Also, Crisis Core and Tales atleast shows there's a market for action RPG's on PSP - Something the sales of those other games doesn't.
As for Epic Mickey and sales, Agent is PS Exclusive even though GTAIV sold much more on 360 than PS3. Agent still has the chance to be a success if made and marketed well. Gears this gen, Halo last gen, and MGS1 in the 5th gen show that exclusivity can be beneficial to getting an IP to really take off and creating their own fanbase.
Do people who play KH expect an exact replication of the Disney worlds? Of course not. Actually, don't most people who watch movies watch it for the core plot and climax anyway? Even then, you can spend more time on each world than their respective movies and there's a large variety of worlds too. The enemies you fight are original, but does that really matter? The funny thing is that all those original enemies and such are owned by Disney and not SE. Org XIII, Heartless, Nobodies, Keyblades? All Disney property.
Crisis Core and Tales show there's a market for action RPGs on the PSP, of course. But I'm not saying any different. I mean, I can easily say Tales on the Wii and Zelda show there's a market for big, grand adventure and action games as well in that case.
We haven't even seen anything about Agent. And it's not GTA. Epic Mickey is a Disney game and so is KH. They're different but at least at this point, Epic Mickey and KH are more similar than Agent and GTA. Also, Epic Mickey is a game that they want to redefine their biggest icon with. Pretty big plans. Agent isn't really redefining anything. Plus the PS3 isn't getting the GTA IV DLC.
Edit: Epic Mickey is described an Action/Adventure/RPG game which is basically what KH is too.