There is no way 2019 will be peak year for Switch. It's still in caterpillar mode, but with the software just announced it is ready to come out and start being a butterfly soon. Switch has an insanely good 1st party lineup this year. It's going to sell over 20m this year, but there are still some things that are going to make later years sell even more (peak 2020 or 2021).
1) Nintendo is only projecting 18m this fiscal year, even though there software is obviously a lot better than last year's. They are going to be short supplied during holidays this year and probably for a few months afterward. This alone is going to keep 2019 from being the peak year, but there are still other factors.
2) Third party games are just getting started and are going to ramp up. Switch is going to keep getting unique exclusives: MUA3, Dark Crystal Tactics, Astral Chain, Trials of Mana, etc.... Those games are just a small taste of what is to come. Also Witcher 3 is a 32GB cart. Expect more bigger games to get ported over like this. The third party flood of games is still coming.
3) Still no price cut or hardware revision yet. There are a ton of handheld gamers still not willing to shell out $300. These will come over when the price gets low enough.
4) PS5 and Scarlett are going to give Switch a sales boost. Most gamers are a lot more price conscious than the people on this board (or at least their parents are). Once they see the choice between a $500 console with a few games and a $200-$300 Switch with a ton of games, then they will buy a Switch. Switch doesn't have this benefit yet, because during the holidays PS4 and XB1 are cheaper and have the bigger library. Once PS5 and Scarlett release, then this advantage will go to the Switch.
5) Expect Nintendo to keep making more first party games. Since all of their resources are on one console they will both revive old franchises and develop new IP. Switch is barely over the 2 year mark and still has a ton more first party games to release. Believe it.
Overall, Switch is just getting started. It has a lot more power cards to play and Sony and Microsoft are planning to release powerful, pricey, systems that are going to struggle until they get a price cut. People have still not realized that Switch is still in its slow phase. Expect it to take off in a few months and not come down again for several years.