The problem is there is still the old DS and the 3DS avaliable on the market for different prices. consumers are probabily confused with what they should get. i think if the DS was discontinued the 3DS would be doing a lot better because there is obviously still demand for the DS.
and i dont like the DS strategy Nintendo have adopted either - which is to release a new model each year, making your model obsolete. people are probabily a bit annoyed after just having upgraded to a DSi Lite, then having to upgrade to a DS Xl that they must now upgrade to a 3DS - takes the piss - which is why i'm not getting a 3DS.
however i think the Wii is the most successful console not having been manufactured by Sony, so i dont think they have anything to worry about on that front. and the Wii2 is coming at a good time, since the 360 and PS3 will have been out a good degree of time and their age will begin to show by the end of next year imo.
