There's one major flaw - you completely ignore overlap. How many Wii owners were also DS owners? How many Gamecube owners were also GBA owners? N64 and Gameboy?
With the Switch, everyone that previously bought 2 different consoles now only buy one.
One more thing - the Switch is likely to remain the most expensive Nintendo handheld in a long time. Now, you can certainly sell a lot even with a higher price, but (and this is mostly based on speculation) - I believe quite a lot of Nintendo handheld sales are from people buying it twice or even thrice. I have two main pieces of evidence for this. The first being merely anectdotal, it's simply that everyone I know who has ever owned a nintendo handheld has owned multiple models of that exact same handheld. The second piece of data that suggests this is Nintendo handhelds and handhelds in general having ridiculously low attach rates. One possible explanation for this would be that quite a large part of the userbase double or triple dip on hardware, but don't match that with an increased software purchasing. With its price being higher than any other Nintendo handheld, fewer people will double or triple dip.
I would say not too much!? GC sold 23m while GBA sold 81m, Wii U sold 13m while 3DS will be around 75m at end.
Switch will definatly have multiply revisions and difrent type of hardwares, so people will also buy multiply Switch.
Also, Switch with big 3rd party AAA games in full handheld mode can be and alredy becoming quite popular and for people that are not Nintendo fans.