But that's not what they're saying. They're developing their next generation system, and they're saying they are. The reality is that most game companies start thinking and experimenting with their next system pretty much when the current gen system comes out. This means that Nintendo's R&D has probably been working on this for around 5 years at this point, and is just now ready to start talking about what they're working on. I wouldn't expect them to actually release a new system until a competitor announces they're making something that might actually have a chance to compete with the DS heavily.
Similarly to how long it was before they updated their Game Boy hardware, they aren't likely to update the DS hardware until it's really absolutely necessary, but they want to have something ready when it is necessary.