Looking at the game's Fiscal Year sales since release, (Special shoutout to @RolStoppable for providing the data , we see that Breath of the Wild has done this much thus far:

FY 03/2017 - 2.76 (2.76)
FY 03/2018 - 5.72 (8.48)
FY 03/2019 - 4.29 (12.77)
FY 03/2020 - 4.64 (17.41)

All signs point to Nintendo taking their time with releasing a successor, so the Switch still has plenty of life ahead of it. This is just a shot in the dark, but I could see future sales looking something like this:

FY 03/2021 - 3.70 (21.11)
FY 03/2022 - 4.00 (25.11) - I think Breath of the Wild 2 will release within this fiscal year, which will give a modest boost to the first game because you'll have people who want to play the first game before jumping into the 2nd one to understand the background behind it.

FY 03/2023 - 3.20 (28.31)
FY 03/2024 - 2.70 (31.01) - I think the Switch's successor will come out here. 7 years after the original released.

Keep in mind the Wii U version was at 1.67 million the last time Nintendo provided those figures. I'd imagine it would be around 2 million by the time Switch to comes out, (If it isn't already), bringing it to between 32.68 and 33.01 million

From there, it will only need an additional 1.99 to 2.32 million units to reach it. Which I think it would easily accomplish if that's the case.

You've gone too far this time you madman.

I can see 30 million happening but not sure about 35 million.

User Requested removal.

the WiiU version has done its duty at 2mil.

the Switch version will definitely pass the 20million mark by the end of the year. over 25 is a given at this point but passing 30mil is all up to when BOTW2 is going to be released.

 BotW2 will push sales not make them stop. Also like SMG to SMg2 I’m betting on BotW budget release

Now I'm going bold to predict 25m LTD and defy Tbone.

Wow used the good ole time machine. 35mil sales. Damn I suck now at Bold predictions but I was on a roll before 2021.

33.64mil NSW sku
1.6mil+ WiiU sku

35mil+ (excludes NS2 sku)

Already did it when you combine both Wii U & Switch, but there's definitely a good chance of it passing 35m on just Switch alone too! (Especially with how digital sales of the NS2 Edition are counted as Switch 1 sales.)