@Sc
well one other big difference between DS and Wii is that the DS can be used as a cheap portable language translator and many many other useful apps that are perfect for traveling. As you might know, language translation is huge in Japan, as even a lot of things in Japanese cannot be translated properly without the right pronunciation that is not always known to everyone :P.
But, yeah of course, while my fun little assumption may keep sales up this year, it will definitely affect future sales in a year or a little more. The really fun part of this scenario is guessing when the perfect time would be for them to both stop making games and at the same time keep sales on an upward trend.
So, from here they could keep upward sales for about a year and then level, but in a year maybe they could last a year and a half before leveling ^ ^.
