Until the semiconductor shortage finally gets sorted out, the Switch will sell through every unit Nintendo is able to order, so it'll be really hard to gauge the actual demand because of it, but it's still got to be pretty damn high if it can ship nearly 11 million units in its 5th holiday season.
If Nintendo ordered enough, with the software lineup it has planned, I could see the Switch being flat next fiscal year. It'll likely be down, but not by much. FY 2022 should be another one above 20 million. And if the shortages are really sorted out by early 2023, if it has another solid lineup, another revision, and perhaps even price cuts across all models, I think FY 2023 could also be >20 million.
They certainly have what it takes to pull it off, but they have to play their cards right and not screw it up by launching its successor prematurely. The Switch 2, which will likely be backwards compatible, should launch, at the very earliest, March 2024. I actually think Holiday 2024 - Spring 2025 is the most likely landing spot. But that should give Switch 1 at least two more full fiscal years to sell on its own.
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