Hard to say. With all of the extras it has, I'd rather see it go beyond what the DS has if that's not too much. And game wise, with what we're already seeing so early, I'd love to see what they could do with the system in the same amount of time or more.
But market wise, no idea. Lately everyone's trying to shift back to the Atari days of quick and easy cookie-cutter games on cellphones and iPod Touch/knockoffs. Maybe that'll help spur on another crash and allow Nintendo to pick up the pieces again, or maybe the aspect of 3D will give Nintendo a quick surge of new/returning customers. Or maybe Apple will try to steal Nintendo's thunder by announcing a newer/more-expensive 3D iPhone/iPod Touch(despite Nintendo's own annoyances with a touch-enabled 3D screen getting smudged easily, as well as being difficult/impossible to make it 3D in landscape AND portrait orientations....but they could still attempt it in some other way I'm sure).
But yeah, mostly, if I ever get enough money to invest in a 3DS, I'd love for it to last as long or more than the current DS series, which was pretty amazing after Nintendogs took off. I think the possibility of a PSP2 trouncing 3DS' visuals immediately might finally give Sony an edge, but that didn't make too much difference this generation. It's just a matter of what Sony decides to 'take' inspiration from this round I guess(such as their own motion-control, touch-screen, camera(s), or 3D-screen).