For what its worth, Nintendo did not have the must have titles until late in the year. At least with 2017, Splatoon 2, Zelda, Mario Kart 8 Deluxe were the big titles that carried the interest and momentum throughout most of the year despite the stock issues. For this year, even though Mario Tennis Aces and Super Mario Party were return to form, Octopath Traveler was a surprising success, and we got a plethora of Wii U ports, those types of titles were not expected to help increase hardware. I get you mentioned Labo, and I was hoping it would help expand the audience, but even then it was a wild card that a good amount of people were betting on.
As for the bolded, was Let's Go supposed to sell all of its initial shipments at this time? It's likely for the holiday season in general as sales-wise, its doing good if you ask me. Especially if you want to give it context, such as this is the first $60 mainline game ($100 w/ the Poke Ball Plus), mixed fan reaction pre-release (though I'm not sure how the Japanese audience thought of Let's Go pre-release), not a next gen Pokemon game, it's on a relatively smaller user base compared to the 3DS when X/Y came out, etc. And despite that, Let's Go managed to pretty much double its COMG numbers in its FW in Japan and sell 3 million WW, for what its worth. Likely netting more revenue for Nintendo overall compared to even Pokemon Sun and Moon (despite S/M selling like gangbusters) because of the Switch itself being a more expensive product than the 3DS in 2016.
I guess though, an on-par or bad second year won't dictate the trajectory of a console's overall LT sales. Things can happen. One of the reasons the PS2 continued selling well, in its twilight years, after generation 7 started was because the PS3 was having a hard time getting off the ground. The 3DS was able to start gaining momentum after a disappointing start once Nintendo slashed the price significantly (which I'm not sure if people expected that at the time). 2019 can potentially be the Switch's best year as Nintendo has more original first party titles waiting in the wings and are not just relying on ports and Labo to carry them until Pokemon and Smash came out. In 2019, we at least have Fire Emblem Three Houses, Luigi's Mansion 3, Animal Crossing, Daemon x Machina, Town, Travis Strikes Again: No More Heroes (not first-party but exclusive), and Pokemon Gen 8, plus a plethora of notable ports such as Resident Evil 4, Final Fantasy VII (on a Nintendo console for the first time ever!), Dragon Quest XI S, and NSMBUD.