Here's my forecasted sales by year
End of 2021: 103 Million
End of 2022: 122 Million (19 Million Year)
End of 2023: 136 Million (14 Million Year)
End of 2024: 143 Million (7 Million Year) Switch 2 Launch
End of 2025: 146 Million (3 Million Year)
Total: ~147 Million
My prediction seems to be the slightly more pessimistic case of a drop off and the Switch still reaches close to 150 Million. Anyone who says switch will sell under 130 Million clearly doesn't know what they're talking about. Unlike alot of people im expecting the switch to fall just short of 20M in 2022. I'm expecting a bigger drop-off in 2023 since that may be the year where rumors and a switch 2 announcement are coming out and we may see a slower 2023 lineup as Nintendo is preparing for Switch 2, so I expect a pretty gradual decline. Than 2024 I expect switch 2 to release but interest in the switch may still be there since it is cheap with a killer library, plus Nintendo may not want to drop the ball on the Switch too early just in case the Switch 2 doesn't work out, kinda like how the 3DS was to the Switch. So I expect a big decline but still a solid 7 million. 2025 and after most people will probably already move on from the Switch as I expect another 4 million before all said and done, bringing the grand total to 147 Million, falling just short of DS/PS2 but easily within reach of those two.