I'm not much of a fan of Nintendo's console design policies, but you're wrong.
PS2 was actually the weakest console of its generation in a lot of ways. It just had a whole bunch of really talented developers who pushed it to its limits, which meant it had really good-looking games. Gamecube was unquestionably more powerful than it, though, which was demonstrated by stuff like Resident Evil 4.
The Wii is a Gamecube overclocked by half in both CPU and GPU, which puts it above anything from last gen, Xbox included. It's still far closer to last-gen than current-gen, of course, but that extra clock-speed gives it an advantage in a lot of areas, including the amount of polygons they can use on screen. (Note that the PS2 had an absurdly high theoretical poly ceiling, but never actually reached it at any point in its life) If Squenix ever bothered to make a full-budget FF game on it, they'd probably be able to make XII with X's character models or something.
...the problem is, of course, that 3DS is not a portable Wii. It's barely a portable Gamecube, particularly in terms of displayed polygons, and what it does have is halved due to the need to render each frame twice for the 3D. KH3D is far closer to BbS than KH2 in terms of stuff-on-screen because of that, and even KH2 didn't have many NPCs.
Then again, Type-0 does this, even on PSP:
Which really does make me wonder what's stopping the KH team from doing the same. =/ (On the other hand, I'm quickly becoming convinced Type-0 was made using crystal magic)
I have an answer for you. The reason Type-0 has many NPCs is because the battle stuff and the "NPC stuff" are separated. Put it another way... you won't see NPCs during battle, the battle engine is loaded separatedly from the NPCs or anything else. Probably if KH had too many NPCs we would get major lags and FPS drops due to the high amount of content the console has to render.
Just look at BBS! BBS lags even at some points with just 3 or 4 characters on screen, imagine how the console would be forced to render all the enemies, NPCs, interactive map stuff, chests and more, all at the same time. Want it or not the NPCs would be big load to the CPU. Plus, the 3DS CPU is clocked to an even lower speed than the PSP's (however, the graphic "card" has the double of memory than the PSPs, so that might help).
