Nintendo Switch has sold-in (shipped) 14.86 million, 3DS 72 million, Nintendo's FY 2017 Nine Months Earnings

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Switch should get at least a 6 year lifespan; replacing it any sooner than 2023 would be a mistake.

And before people bring up "but Switch will be so outdated by then", see DS and 3DS, which kept going long after they'd become technologically ancient.