The Switch will continue momentum for a while yet. Pending on when BotW2 lands, 2020 could easily be the peak year. However Pokemon in 2019 will make it a hard year to beat, so I still have my doubts that 2019 won't be the peak sales year instead.
When the new consoles drop, there will not be as big of an impact that many want to speculate. Switch is the only console that offers portability so those that are looking for that will still flock to it. Not to mention we have yet to see any hardware revisions, which will also boost sales. Thats not to say Switch will be unaffected entirely, but it will still easily cross 100mil LTS.