In a 15 month period ranging from March 2017 to May 2018 Switch sold 17 million units, while XB1 sold 11 million. So Switch has been outselling XB1 by about 400,000 a month.
http://www.vgchartz.com/article/277036/switch-vs-xbox-onevgchartz-gap-chartsmay-2018-update/
Based on the 400,000 a month gap, I expect Switch to sell about 10 million more units than XB1 from July 2018 to November 2020.
XB1 sales should decline a bit, because people will be expecting XB2 winter 2020. After 2020 Switch just has to catch up to a dead horse. So, by December 2021, I expect Switch to surpass XB1.
Last edited by Cerebralbore101 - on 10 July 2018