With PS5 + XB2 launching in 2020.... I think 2019 is the "peak year" for the Switch.
However that doesnt mean its sales will fall off a cliff in 2020, though I suspect by 2021 its sales will have dropped alot.
I actually doubt that happening. Nextbos and PS5 are launching holidays 2020, so most of the year goes on pretty normal. Switch now has in the second half of the year 2019 a lot of releases and with Animal Crossing that goes on into 2020. So the library 2020 is pretty great. If Switch is dropping in 2020 it will not be because of the other consoles. Similar to PS4 not really have been impacted by the Switch. Switch release impacted Vita and 3DS, but PS4 stayed it's course. In the same way I think Switch will be not influenced much by the others. At least at first. Later on in 2021 and 2022 bigger next gen games will come to these platforms, which will not hit Switch. At that time Switch may start to look old.