I've always said it's silly to combine Nintendo console + handheld and think of it as a distinct userbase.
Probably upwards of 70% of Wii U owners owned a 3DS too as they were the core Nintendo fans mainly. So you can't just add the two groups and treat them like a distinct audience. It's really more like 3DS owners + maybe another 5-6 million from the Wii U and that's it.
And selling as much as the 3DS in Japan ... if you gave Nintendo that option, they'd take it and run. 3DS outsold PS2, Wii (easily) , Famicom, Super Famicom, in Japan.
Matching 3DS will be tough in Japan because Japan has become increasingly smartphone/tablet centric the last couple of years (I think it was the no.1 consumer of smartphone games a year or two ago). So if Switch can keep pace with the 3DS in Japan over the long haul, Nintendo I imagine would be very happy with that given that Switch is entering a tougher market than 3DS did in early 2011 (smartphone games weren't as prevalent then).
