I still feel that compared to the dual system of Wii and Ds which sold 250 mill combined, Nintendo have made a mistake. Even 3ds and wiiu combined sold 90 million which is not that far of switch. Switch has been a success no doubt but compared to that dual home and handheld market Nintendo are missing sales.
I wondered how long it would take before someone brought up the Wii/DS generation, this was pretty much lightning in a bottle, two new techs in their infancy in the home markets, touch controls on the DS and motion for the Wii, anybody could see the casual nature of these audiences, and those customers were simply not around then the next generation came along with the Wii U and 3DS, and as you said sold 90 million combined.
Also the dedicated handheld market has been in decline, this is due to tablet devices and smart phones, the drop from the DS is stark, as was the Wii To Wii U, hell even Sony realized there was no market for them in the dedicated handheld space after the Vita.
So you have to consider the extra costs of developing two individual consoles, and the fact both will have product revisions at some point, more so on a handheld type device, so Nintendo have not only sold more hardware units than the Wii U and 3DS, they has saved a fortune just developing one system, far from a mistake in my book.