Because the new model isn't releasing until September and between now and then sales are going to slow down especially as we get close to September 20th, it could even dip below 2017-18 levels right before the Lite comes out. And it's going to be starting 2020 riding high off of Sword/Shield plus the new model, much like the Switch was riding high off Let's Go and Smash Ultimate entering this year. That eventually that momentum will carry it to Animal Crossing: New Horizons which will only build upon that momentum before it fades off. Pokemon + Animal Crossing + a cheaper handheld model is going to make a lot of handheld-only gamers and parents jump on board when they wouldn't before ESPECIALLY in Japan where weekly sales were starting to go down week over week before Mario Maker 2 launched. Depending on what else Nintendo has slated in 2020 along with that game, it could lead to to sales getting consistently higher. January through late September could see the Switch being consistently up YoY for 2020 until the Lite launch effect kicks in, then who knows. 

Wasn't expecting the new model to be $100 cheaper, nor was I expecting the new model to be a different size. I thought this years model would be a simple revision with the new Mariko chip, increasing battery life and reducing heat/noise. Which while nice wouldn't have driven sales much. Instead we got a different form factor at a substantially cheaper price.

I'm still 50/50 on whether 2019 or 2020 will be the peak year though. After all Nintendo did only forecast 18m shipped this fiscal year, though I'm sure they'll exceed that quite substantially. 

But I imagine now everyone who wagered on 2020 in this thread is sweating a little.

I don't know why their forcast is 18 millions with a brand new and cheaper hardware, and a mainline Pokémon game

I bet on 2020, but I could see 2021 also being peak year. No way is this year going to be peak year, even though they are going to blow past the 18m projection for the current fiscal year.

Plot twist: there'll be no peek year

With the new model it certainly got more interesting, but which year it will be is still a pretty open question in my opinion.

Will still be next year.

With the announcement of Switch Lite I'm confident it will be 2020.

Why? After all, it comes pretty late in the year and has been announced early, which can result into a sales gap.

On the other hand, the new model will certainly boost the sales throughout the 1H of next year. If there's some good titles during the second half to keep the sales high, I don't see why 2020 couldn't beat 2019 in cumulative sales. 2019 holiday season will probably be higher than 2020, but I'm not nearly so certain for the whole year.