Currently Online 
I really doubt that the Switch is going to be comparable to the N64 or GameCube. With those consoles, many of the biggest hits were released within the 15 or 18 months of the platform's life (Super Mario 64, Mario Kart 64, Goldeneye, Melee, Super Mario Sunshine, Wind Waker, Luigi's Mansion, Animal Crossing, etc). Not only does Nintendo seem to be doing a better job at timing its releases on the Switch, they are going to have less distractions to divert developer focus (the Virtual Boy, N64 Disk Drive, N64 peripherals, the early DS launch, etc). The DS has what might be the opposite problem, since it had to compete with the GBA and the PSP for a while before really taking front stage.
Numbers more like the SNES or Game Boy would require a game-changer latter in the Switch's life, like Donkey Kong Country and Pokemon. This is entirely possible, but can't be counted upon.
The 3DS and Wii patterns seem entirely possible, seeing how Nintendo will support the Switch heavily through 2019 and keep it around for a full cycle, unlike the Wii U and GBA.
This suggests Switch lifetime sales circa 50 million.
If I had to guess, I think Switch sales will be somewhat higher than that. From what I know of the hardware, the Switch uses a lot of mobile components, which seem to be constantly falling in price or having breakthroughs. That, along with the mid-life redesign phenomenon that works great for handheld systems, can give the Switch a second wind at some point. This sort of thing HAS happened before with the DSi and Xbox 360.
