I think 21-24 might be a little too low, for the Switch to somehow land in that sales range, it either needs to have 0% YoY growth for every week for the rest 2020 or start having a significant YoY decline in the Holiday Season which I find difficult to believe will happen unless there are major shortages.
The Switch sales in 2020 Year to Date compared to 2019 is already nearly 4M units ahead of 2019, so the Switch would essentially need to have a 0% growth or a decline for the rest of 2020 to land between 21-24M sold for the year,which let's be honest, it isn't going to happen since week after week during this quarantine Switch sales have DOUBLED from 2019, to think that the Switch in the future anytime this year would go week after week having a decline or 0% growth at this point is unrealistic.
You brought up the fact that the majority of the Switch's sales came from the Holiday Season 2019 which may be hard to top, which is true. However, based on how much demand there currently is for Nintendo Switch's and even before this Quarantine and Animal Crossing Switch sales were up 23% YoY, it's hard to believe the Switch's 2020 Holiday Season will sell any less than 2019 since there's more overall interest in the Switch and this quarantine will only help spread awareness of the Switch even if it ends by the Holiday Season.
I believe 24M sold is the min for 2020, unless shortages become a problem
You may be right. At this point it is somewhat of a shot in the dark, because we don't know what kind of games they will have for the rest of the year. Also there is still a chance of a "minor" title becoming a surprise hit like Clubhouse Games.
However, what I do know is that the last 1/3 of the year was extremely strong for 2019. Switch Lite + Pokemon + a bunch of other titles gave Switch a strong boost. I am skeptical that Nintendo will have a lineup anywhere near that good during this holiday. So, given their strong current momentum + a weaker lineup for the rest of the year, I think an assumption of them being close to flat going forward is not unreasonable.
Of course if they have BotW 2 and another strong game this holiday, then they will probably sell even more, but I am skeptical their holiday titles will be as strong as last year.