Combining both versions, I definitely think it'll pull it off. MK8D is shipping 1 million copies every quarter, and that's going to be way up this quarter thanks to the holidays.
Mario Kart 8 Deluxe's shipment data per quarter: Quarter (Total)
Q1 FY 2017: 3.54 million (3.54 million)
Q2 FY 2017: +0.88 million (4.42 million)
Q3 FY 2017: +2.91 million (7.33 million)
Q4 FY 2017: +1.89 million (9.22 million)
Q1 FY 2018: +1.13 million (10.35 million)
Q2 FY 2018: +1.36 million (11.71 million)
Q3 FY 2018: +3.31 million (15.02 million)
Q4 FY 2018: +1.67 million (16.69 million)
Q1 FY 2019: +1.20 million (17.89 million)
Q2 FY 2019: +1.12 million (19.01 million)
With ~10 million units to go & the legs it has, it can ship between 4-6 million units a year and can pull it off some time in 2021.
As for the Switch version, by itself. That's going to be a much tougher task. It needs to ship another ~18.3-18.5 million units to pull it off. That's going to be extremely difficult to accomplish, even for Mario Kart. I'm going to take a "wait and see" approach for that one.