I'll be optimistic and say that MK8D can pull it off alone in the end unless Nintendo comes with a MK9 before 2021. And unless Nintendo brings out MK9 next year already, beating MKWii by combining Wii U and Switch numbers are already in the bag.
I mean, according to PAO's post MK8D sells about 7M a year on Switch right now. I do expect the sales to slow down a bit with time, but by end of 2021 it should sit at only 2-6 Millions from MKWii (3M this holiday season plus 5-7M per year in 2020 and 2021) unless a new one came out in between, which would drastically slow down the sales of MK8D.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454 List as Google Doc: https://docs.google.com/spreadsheets/d/1aW2hXQT1TheElVS7z-F3pP-7nbqdrDqWNTxl6JoJWBY/edit?usp=sharing
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/
Free Pc Games thread: https://gamrconnect.vgchartz.com/thread/248138/free-pc-games/1/