I'm not sure but I think a lot of people are underestimating the Switch's sale's potential just because of the Wii U's sales.

I love to be wrong. So, I say the Nintendo Switch will sell less than 50 million units.

The Switch has an advantage over Wii using mobile tech, and will get HW refreshes like typical Nintendo handhelds to keep its market presence strong..We will see dockless Switch "Lite" (no joycons, resembles vita, dock sold separate with controller) New Switch, etc. So worst case scenario for Switch is just past 3DS or 75 mil, and best IMO is above Wii / OG gameboy total.

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The Wii was the best selling console for the first four years of it's life. 

For context, year 5 was better than any year XBone has had to date. Year 6 beat any year the Wii U had. (not hard obviously) 

I feel like the Wii has a reputation for being a flash in the pan, but 20 million people came back for Wii Fit Plus. 30 million came back for Wii Sports Resort.

It had a longer life than people seem to remember. 

It depends. If Nintendo stops supporting the 3DS within the next year or two and makes the Switch the sole home for Pokemon, I could see 100M as a distinct possibility. They really need to double down on this hybrid concept: a system that's both a home console and a high-end portable, combining all of Nintendo's franchises into a single platform. Improving the Switch's battery life will also help with that.

They will pass the 50 million mark for sure. But 100 million will be difficult. I guess we will see some revisions until then...

2006: 2,954,781

2007: 16,553,289

2008: 24,188,263

2009: 21,295,417

2010: 17,320,956

2011: 11,522,685

2012: 5,082,024

2013: 1,947,850

2014: 515,969

I was still playing on my Wii occasionaly in 2012/13, when even though the Wii U was out, I never knew it existed at the time.

I agree, this is a big difference. There will be lots of different Switch models and IMO not just cosmetic differences. I think Nintendo will have different classes of Switch hardware as time goes on. As such they could have more repeat buyers. It's simply good business. 

Though I don't think everything should be compared to Wii. Switch is a very different system, why is Wii always held as the gold standard? It's not even one of the three best selling platforms, and DS destroyed it in sales too, but aside from that there are obviously huge demographic/product differences too. 

Switch will succeed on its own merits relevant to today's market (which is radically different from 2006), not by looking backwards. 

Switch will sell more than Wii.