Well, the slot, and probably also the GBC sound circuitry and GBA BIOS, both of which are required for GBA games but inaccessible in DS mode. But yeah, the ARM7, the main processor of the GBA, is fully utilized by DS games. So it has to be there. Unless I'm mistaken, being unable to access GBA BIOS while in DS mode is why Slot-1 GBA emulation is basically unplayable.
Reminds me of how GBM features a Z80 (GBC BIOS and all) despite lacking the GBC voltage switch, although in that case it was only because Nintendo didn't bother making a new chip set for it. GBA games don't use it.
Axing GBA support (and all accessories and features that came with it, including Band Bros expansions, Rumble Pak, Guitar Hero, and Pokemon's Pal Park) is not worth a higher price, low-resolution camera, shorter battery life, and a bunch of other crap that homebrew accomplished ages ago. Hate the AAC format as well. That being said, Nintendo would be utterly foolish to cut off their massive installed userbase from the download service. One hopes they have an adapter for older systems in the works. Something akin to the DSVision, perhaps?