It's a DS game, first of all. And the thing about sequels to established franchises is that the sequels are usually bought by people who already own the original so no spike in hardware. (unless Nintendo does a 3DS pokemon bundle)
There will be a spike - its likely to be the biggest release this year and some people will use it as an excuse to get also a 3DS if they dont own one. There are still many out there who own a DS plattform without DSi capabilities.