Hmm this cant be decided if we dont have numbers. I still think that there are maybe 20 Million different people in Japan which have a Nintendo DS people collect it. People break them accidentialy people have one in the car and one at home and so on. Selling maybe is an option to some but I see it like cellphones people have multiple cellphones also some might have multiple DS. And in Japan families are actually shrinking. Not everyone has people to give the old stuff further.
So I say userbase is 20 Million and not 30 because of Multiple owner and probably broke consoles, you are disagreeing the points are good but aslong as we dont have a good survey about it we will never know the truth.