Okay, I'm a little confused here. When you say outsell do you mean the Xbox One's 30 mil sales as of now or it's lifetime sales? Because we don't know what Xbox One's lifetime sales are, and if you're saying Switch will sell 30 million by late 2020 or early 2021 , that's not that impressive.
By comparison, the Xbox One officially reached 26 million sales by January of 2017, it took about 1153 days. If we say the Switch sells 30 million from March 3rd to October 3rd of 2020, that's 1306 days. Not very impressive.
In conclusion : It depends *entirely* on how much sales you think Xbox One will get lifetime, or if you are putting it up against the current 28-30 million benchmark.
I mean when will the Switch pass the Xbox One. So if the XB1 is at lets say 40mil by 2018 and the switch catches up and sell 40il+1. So i think end of 2020 XB1 will be around 55m and Switch 58m.