I imagine therein lies the rub. If you market to people who don't traditionally buy games the likelihood is that they'll not be in any great hurry to buy the next Mario/Sonic/FF/Whatever game, and will be quite happy with the games they got at launch for the occasional times they dig out the system. That's why I predicted a while back that the PS3 would eventually take over.
Nintendo probably needs to capitalise on its high market value and buy in some franchises that will appeal to the games playing (and buying) masses. It would also be an idea for them to be working on the next console rather quickly, OR "doing a Sega" and supporting other platforms once demand for their own dies down.
Of course the DS is likely to sell on and on and on!