I don't really think you can establish any sense of normalcy based on the pre DS Nintendo handhelds. The Gameboy had an extremely long life with a late life resurgence thanks to Pokemon/Color. The GBA on the other hand rocketed out of the gate only to be replaced by the DS early in its life. They were also both near their sales peak at the time their successors rolled out. It is very unlikely for the 3DS to be in a similar situation when it is replaced.
Just averaging them out doesn't make that figure "normal". You could easily fiddle with the numbers to get other figures which could be called normal. In the 7 fiscal years prior to the year of the DS launch, Nintendo shipped 116m handhelds. That averages out to 16.57m per fiscal year. Nintendo hasn't shipped that many for the last three fiscal years. This past fiscal year wasn't even close, and it seems unlikely they'll return to those levels soon. Their best shot is probably the first full fiscal year of their new handheld assuming the 3DS hasn't completely collapsed by then.
