End of 2017
End of 2018
End of 2019
I don't think Switch will do 20mm in 2019. Though many systems peak in year 3 or 4, I think the poor performance of the Wii U actually pulled Switch sales earlier into the generation. People were hungry for new Nintendo hardware. Also, they used two of their big guns early - Zelda and 3D Mario (I know Zelda isn't generally a huge system seller, but the incredible reviews of BotW likely helped it move more units). One of the other big guns is being used this year - Smash. There's also that Let's Go Pokemon thing that will bring many of the core Pokemon players into Switch this year, if they're not already owners. That means next year's traditional Pokemon won't have the same system selling power that it otherwise would have. After that, we have Kart 9 and the next 2D Mario. But, fans of those games are probably already owners. With all of this said, I'm thinking 2018 may be the peak sales year.
I should point out that I'm not predicting a massive decline in sales after 2018. But, it may be in the 15-18 million range next year. If so, it won't catch XBone until 2020, possibly late in the year.
Price cut, revisions & big hitters like 2D Mario, Pokemon Gen 8 & Animal Crossing are things that i could see causing Switch to have a stronger 2019.
But ya its possible 2019 sees a decline causing Switch to not pass XBO until 2020, i could see that happening as well.