The real question is will it do it next week.
That would be a massive 2.2 million units sold in a week. What was the 2018 sales of the final week of the year?
50 - 48.8 (current) = 1.2 million to go. Based on this week's sales, I would say it has a VERY good shot of hitting 50 million by the end of 2019.
55 million by the end of March should be pretty doable as well. The only big Switch game I'm aware of on the schedule is Animal Crossing: New Horizons on March 20, but even without anything in Jan and Feb I don't think it matters too much. Switch can easily ride the Pokemon hype wave for the first couple months of the year to keep the strong sales going.