By the time the PS Vita will be released in all regions and its post-launch game drought will be comming to an end the 3DS will have seen a price reduction, will potentially have a re-design that adresses most of the issues people have with the hardware, Nintendo and third party publishers will regularly be releasing interesting core titles, and Nintendo will start releasing market-expanding titles on regular intervals.
At about this time, DS interest will begin to drop off rapidly and people will be looking to upgrade to the next generation, and backwards compatiblitiy, the lower price point, a larger library, and the steady release of titles that interest DS owners will enable Nintendo to recapture the bulk of these users.