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.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071