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.
Just as I've always said for Switch vs PS4 though, they're not in direct competition, which is why PS4 had it's sales peak the year Switch launched. They don't affect each other in sales, and PS5/XB2 isn't going to affect Switch in sales to a noticeable degree either.
I expect the Switch to have a 20m+ year, and it looks like 2019 might not be it, so I think 2020 will be the peak and sell 20m. Either 2019 or 2020 is going to be the peak year though.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)