I think a lot of people may never recognize that the massive success of the DS line was due in part to the DS tapping into a market that until the rise of the modern smart phone, had little competition.
I always got the impression that the DS enjoyed unmitigated domination because nothing else was really servicing the audience for casual and "friendly" gaming in a handheld format. Its sales got some degree of boost there.
Because of the mere fact that there's more choices in the market today, it may be inevitable that a DS phenomenon can never happen again to the same degree. But by the same token, it doesn't mean DS-like products will now fail or be extremely niche.
I'd predict the 3DS has the best shot at literal DS equivalency in Japan, where it can best take advantage of unique interests in a small, concentrated market. I wouldn't be surprised at all if it sells every bit as much as the DS did, in Japan.
Elsewhere, it may never reach DS numbers, but if it doesn't, that should be seen as natural. The DS was ahead of the curve, exploiting a market that wasn't fully understood yet. Trailblazers don't always dominate successive waves of colonization.
But there's no reason why the 3DS can't do very well, in other markets. That brings us back to the likely fallacy about "everyone just wants smartphones lol" perpetuated the cult of iOS pundits.