The only thing I worry about is the 3rd party support, especially on the side of the higher tier Japanese developers. With all the success stories surrounding Switch, companies like Capcom are kinda dragging their feet when it comes to supporting Switch with new games (not ports of old games). Games like Granblue Fantasy Vs., Guilty Gear 2020, Project Sakura Wars, etc. are not coming to Switch or even hinted of possibly even being ported to Switch. I know there are games like NMH3, Digimon Survive, and the Trials of Mana remake coming to Switch, but I think there is potential for more, especially now that the userbase is going to be more than sizable enough that bigger companies can't ignore.
Capcom can't be helped and the same will probably continue to hold true to varying degrees for the other big publishers. Square-Enix has at least committed as much as keeping the Octopath Traveler studio on Switch games and their Tokyo RPG factory is multiplat anyway; chances are that they start to dump Kingdom Hearts ports on Switch too. Dragon Quest XII should be a given. Atlus has stuck with the 3DS for a long time, but they obviously have to move on.
Overall, I don't think these publishers are important enough to really harm Switch with their lack of support, because the smaller publishers have already included Switch in their game development. Support will improve over time, but third parties on the whole won't make use of the full potential that's there.
That's not how console sales works. The 3DS had only Super Smash Bros. for 3DS left after Pokémon X/Y as all other big Nintendo IPs had already been released for the 3DS, but another ~11m in hardware sales got added to the ~13m that had been sold by the time X/Y launched. In other words, there's no need to worry that Switch could sell less than 20m lifetime in Japan, because that's simply not going to happen.
Pikmin doesn't do anything worth mentioning for hardware sales, so it shouldn't even be brought up.
The line of thinking that "nothing is left" doesn't make sense for two big reasons:
1. Sequels to big IPs also push hardware sales.
2. Nintendo can create new IPs that attract new players. See: Ring Fit Adventure.
1 : WHO really, who doesn't have a Switch, and who was not enough interested by Zelda, will suddenly think : "omg the new sequel of Breath of the Wild is awesome, I want a Switch !!
well, I have an answer : me, so far I only have the Wii U that I bought in end of 2017 just for Zelda, but we are marginal in this situation.
2 : You mean like ARMS and Astral Chain ?...
1. I refer you back to the post of mine that you quoted. The 3DS sold another 11m units with next to nothing left. That's close to half of its lifetime sales.
2. I named Ring Fit Adventure as an example, so why do you even ask?