If he Switch can stay ahead throughout next year, then it will also very likely outsell the PS4 unless it crashes like the Wii after 2010 - but I don't see any reason why that crash would happen right now.
It won't crash as hard as the Wii, or even 3ds, but it will decline significantly in 2021. Wii in 2011 (33% drop), 3DS in 2014 (32% drop). I expect Switch to drop by around 27% in 2021.
Switch January 2019 to December 2019 = 20.5m (LT 50.1m)
PS4 September 2015 to August 2016 = 16.6m (LT 42.8m)
Switch January 2020 to December 2020 = 22m (LT 72.1m)
PS4 September 2016 to August 2017 = 21m (LT 63.8m)
Switch - January 2021 to December 2021 = 16m (LT 88.1m)
PS4 - September 2017 to August 2018 = 18.8m (LT 82.6m)
Switch - January 2022 to December 2022 = 12m (LT 100m)
PS4 - September 2018 to August 2019 = 17.1m (LT 99.7m)
A few months into 2023 and PS4 takes the lead launch aligned.
It'll only be a little over a year to discover if I'm wrong or right, (or even less if 2020 proves to be a disappointment, though this seems very unlikely at the moment). By Summer 2021 it should be apparent that the Switch is either declining fast, or holding steady.
2021 really is the all-or-nothing year for the Switch.