Looks like you don’t get how Nintendo hardware works. To begin with, Zelda is not even an A tier game for Nintendo when it comes to sales (not even close). Second, it will probably ship over 17m when it turns 1 year on the market (end of february) with all those stock issues throughout the whole year, and he’s asking from March 2018 to April 2019, when we expect shipments to increase even more. It only needs Animal Crossing and good support titles for it to happen, but if by chance Pokémon release in this timeframe... well, the amount it will sell will be only limited by its stock.

Hopefully with localization.

I didn't specifically take part in the discussion but it was about Japan sales over the course of a few weeks.

I'm thinking 10-15m, momentum generally the same (or really a little less).

Nintendo had an extremely good first party line-up it's first year: Mario Kart, 3D Mario, and the best Zelda in several decades, plus several other 1st party titles. Nintendo has Pokemon still, but I don't think Pokemon is coming til 2019. The leap from 3DS to Switch means it will need a little extra development time. Switch will also have much better 3rd party support in 2019.

I don't think 2018 will be nearly as good as either of those years. It still has a couple of other strong titles for 2018 like Smash Bros or Animal Crossing, but I don't think that will be enough to really increase momentum over 2017. Also Nintendo is releasing it's online service this year. I expect it will have some kinks that will have to be smoothed out. I expect the service to be good eventually, but Nintendo seems out of their water with anything involving the internet.

So a combination of less impressive first party games and an initially disappointing online service will kill a little of the Switch's momentum.

2018: 19m-21m
Next Fiscal Year: 24m

Nintendo is planning 20 million. So, seems they will be able to ship the 20 million. And are planning software to achieve that.
They planned 14 million in 2017 and sold a little more.
Either way, I think 18M+ is safe.

Well given the recent rumors lately along with OP's pessimism or realism should I say, I think it's about time to revisit this topic ... 

There's a few fatal flaws that Nintendo has with the Switch that'll keep them from reaching their expectations and let alone reaching performance close to juggernauts including the PS4 which arguably made the least amount of mistakes out of all the current platforms but still probably won't get to the same level as the two beasts. Some of those flaws are just inherent weaknesses and others are their decisions/executions ... 

Let's begin with their biggest inherent disadvantage ... 

Switch's appeal is not enough to overcome Nintendo's weak brand power in Europe and we can see this in comparison to their predecessor the 3DS, as both are sitting at ~2.9M units on the 51st week at the year they launched in ... (honestly just this reason alone is enough to rule out that they can realistically reach their 20M unit target) 

Here's several missteps that Nintendo did ... 

Switch's pricing is not a virtue and people should not argue it to be such. The DS could not have sold as much as it did if it launched at $299 since it discourages multiple unit ownership by household as it becomes prohibitively expensive for a household to own multiple units ... 

Nintendo failed to followup with their 2017 lineup in comparison to their competition. Aside from maybe the rumored Smash Bros remaster the rest of their lineup is not good enough. LABO is not good enough and we can see this on Amazon where Nintendo products are usually over represented that it's getting beaten by older Nintendo titles and it keeps falling further behind in the daily rankings too. Ports of old games being the majority of the lineup is absolutely not acceptable either ... 

Mediocre to bad line-up: 10-15M
Decent to great line-up: 15-20M
With some huge games and a price drop: possibly over 20M

But it's still too early to tell, it seems like Nintendo doesn't want to announce their stuff too long before release.