Nice, that you included other systems time to that milestone. I thought 3DS must have lost steam later on, but looking at the list I realize that 3DS is actually pretty strong in Japan, currently 3rd in total sales (only topped by Gameboy and naturally DS). There are five systems sitting between 5 and six million units in Japan (which Switch should beat this year): N64, Sega Saturn and the Vita. After that is a big gap with only the PS4 in it, which obviously is still a moving target. The next settled target is PS3 with just above 10 million units. Can Switch get there next year? Possible.
Yeah, I also didnt know that 3DS was fastest selling console in Japan (at least to 5m).