Shit, I still remember my original prediction: 80-100 Million.
Reason for that was because of the monopoly of the Handheld market like @Wyrdness said, but also because I assumed there wasn't a complete overlap between Nintendo's Handheld audience and Console audience. That there were some Wii U owners that didn't own or weren't interested in the 3DS that would buy the Switch, and vice versa.
Boy, it wasn't just Wii U and 3DS owners that were interested!
Hardware Comparison Threads:
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PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
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PlayStation 4: 2015 vs. 2016 vs. 2017