2021 is absolutely in the bag for Switch.
In 2022 Switch sales will still be too strong for the PS5 to overcome. This thread is yet another instance where PS fans conveniently forget how good Switch's software pipeline is and that Switch serves both the home console and handheld market. There won't be a sharp dip in Switch sales, especially because Europe and Other regions remain strong growth areas for Nintendo; Switch started with modest numbers there and that in turn means that the saturation point in those regions will be hit later than in North America and Japan, so global sales will remain strong.
2023 is the first year where the PS5 will have a shot to sell better than Switch, but until then it has to be seen if the PS5 really has 20m+ years in it which a portion of PS fans believes. Much of it will depend on the reception of Microsoft's Game Pass and better multiplats on XSX which can both prove to become a notable obstacle for Sony, and Sony is helping Game Pass to get more attention with raised software prices. 17.5m+ might already prove to be challenging if Microsoft's offerings get traction and that level won't be high enough to beat Switch which will have better models and price reductions by 2023. Yes, Switch still selling ~20m in 2023 isn't out of the cards with continued strong software support and incentives to add more Switch units to a household.