I've been lurking on these forums for awhile now, and this thread gave me reason to make an account and start posting.
There are many things to consider when figuring out what console it'll be on.
1. Just because the PS2 had an incredibly long life cycle doesn't mean the PS3 will. This has pretty much been proven by most other console manufacturers. Microsoft pretty much dropped all support for the original Xbox when the 360 came out, and same for the Gamecube when the Wii came out. The PS2 lasted so long because of the slow adoption rate of the PS3
2. The architecture of the PS3 and the PS4 are completely different, meaning cross compatibility will be incredibly difficult. The PS4 will be using legit computer parts, based off an X86 CPU. The PS3 was not, at all. I don't want to get into details, but this will most likely be the reason why the PS4 will not be backwards compatible
3. It'll be a bigger middle finger to people if Versus 13 doesn't come out on the PS3 than if Kingdom Hearts 3 will be. Versus 13 has been in development FOREVER. Sony thought the PS3 would still be going strong, but they didn't count on how fast it would become outdated. Kingdom Hearts 3 hasn't even begun to be developed, so it would make little sense to make it for an outdated console just to make fans happy.
4. People say they should make it for the PS3 because the PS4 isn't selling well and they'll make more money if it is for the PS3. I can pretty much garuntee that once they begin development, Sony will sign a deal with Square Enix to make it on the PS4. Why? Because this game alone will sell PS4s. I know, I switched from being a console gamer to a PC gamer, and I will still shell out $$ to buy a PS4 when Kingdom Hearts comes out. I did it for the PSP and the 3DS, I'll do it again.
This is why I think it'll be on the PS4 and not the PS3