You make some very good points.  However I want to say the most important thing for any console is the games.  A console is just a device you play games on.  It's the games that give a console it's worth.  However the games at launch and also during the first year set the trend for how the console is going to do.  

(referring to bold) In this sense, the Wii and Switch succeeded for the same reason.  They both launched with flagship games that people really wanted.  New gamers really wanted Wii Sports.  Established gamers really wanted Breath of the Wild.  Both devices had great flagship games.  The Wii U had Nintendoland as its flagship game.  People just did not get excited for this game like they did for Wii Sports or BotW.  In that sense it should have been clear that the Switch would do well right out of the gate, because it launched with such a great game.

I don't want to negate what you said though.  The hybrid nature of the Switch gives it huge advantages.  The biggest advantage is that Nintendo always gets good third party support for it's handhelds.  They have had problems getting third party support for their home consoles, but not their handhelds.  Now they can use all of their strength in the handheld market and bring it to the home console market.

That is why the people who are now guessing only at the 100m mark for Switch are still way undershooting it.  The real wave of third party games hasn't hit yet.  Third parties need time to make their games and most of them wouldn't have thought about the Switch until after they saw it succeeding.  Next year we are going to actually start seeing new third party games for Switch, and in 2020 we will get even more.  The Switch is still in its slow phase.  Once a large wave of new third party games comes along, then Switch sales are going to move into high gear.  (And no I don't mean Western AAA, because those games are overrated anyway.)

Yes, it's true, the flagship games at start define a lot of the momentum. But they cannot give it a go alone. The Playstation consoles pretty much sell without flagship games. A friend of me who bought the PS4 at launch literally said, that he got Assassins Creed again, although he had the game on PS3 already, so that he has something to play with at start. Playstation sells without big flagship games at start, because people EXPECT big 3rd-party support and 3rd-parties EXPECT big sales, and so each of these expectations leads to fulfilling the other. So yes, constant stream of 3rd-party can be relevant for momentum. Switch would be massively lower in sales without these streams of often smaller or older titles.

Going buy current trajectory of sales, Switch will be at around 35m after first two years on market.

I think group psychology might have had something to do with it. This thread was made right after the Switch presentation in January 2017, and a lot of people (especially on this site) were extremely negative about it. When people constantly see everyone else talk about how bad something was, they start to get a more and more negative feel from it themselves.

My prediction was 70 million, which may not seem much now, but back then I was constantly wondering if I wasn't overestimating it (considering how low most people's predictions were).

I'll try to stick with my prediction from last year.

Switch:
USA: 30 M
Europe: 25 M
Japan: 25 M
Other: 10 M
Total: 90 M

PS4:
USA: 40 M
Europe: 45 M
Japan: 10 M
Other: 25 M
Total: 120 M

XB1:
USA: 33 M
Europe: 17 M
Japan: 0 M
Other: 10 M
Total: 60 M

I believe it will sell well over 80 million after the 2018 semi-stagnation, 2019 seems already greater and switch has not lost its momentum

I don't fully buy this point of view because even with the presentation which wasn't even that bad tbh I don't know what some people were expecting the was one thing clear back then and that Switch would be the only portable device on the market and all the 3DS and Vita consumers would have to go somewhere and that alone called some of the pessimistic views into question.

I know some of this happens, but that kind of ignores the fact that hardware sales always go up whenever a big game gets released.  One reason that your friend bought the Playstation is because the competition didn't look any better.  Really if another console comes along that either has better games or the same games on cheaper hardware, then most people will switch.  That is what happened in North America with the XBox360.  In gen 6, everyone had a PS2, then in gen 7 they bought a 360 instead.  Now in gen 8 most people switched back to PS4.  These people might have even bought the console before the game they wanted came out, but they were very willing to switch.

In Europe, most people did not switch over, but that is because they weren't offered the right console for them.  If a better competitor comes along then they will "switch" away from the Sony console.

I could see that happening.  I watched the presentation at home by myself, and I was really excited about it.  My reaction to the initial showing of the Wii U was the exact opposite.  I may have benefited by not listening to anyone on a message forum.   Of course on any decently big message forum there are always a couple of people (at least) who work for the game industry, so you have to beware of listening to biased opinions.  Also the gaming forums are often out of step with the mainstream.  It's very possible to go to some forums and get the impression that the Dreamcast was the best console of generation 6.

Wow, those predictions...10-20M lol.

I'm thinking this thing is hitting 80-100M depending if they choose to milk it like they are doing the 3DS. It's going to possibly hit 30M without a real Pokemon game, or Animal Crossing, among a ton of other 1st party heavy hitters and 3rd party secrets. It's got a bright future ahead of it, that's for sure.

I think people should not predict sales based on negative feedback on forums. People here and in other forums like Resetera, GameFAQs, 4chan, and N4G are a specific audience, and relatively small ones from a macro point-of-view. Why did Switch sales INCREASE/IMPROVE AFTER E3 2018 even when some here didn't like the E3 Direct, or at least the overwhelming Smash Bros. Ultimate portion of it? Why is 1-2 Switch still selling despite the relatively mediocre reception and hardcore fans hating/making jokes on the game?

We can all predict and have opinions one way or another, But let's also think a little less emotionally (or use the emotions of people from forums), The Switch Presentation in 2017 did its job on selling consumers the concept of the Switch, the games, the price, the launch date, etc. Sure it still had questions then, and even now, but it built a basis on what to expect out of the package and what to look forward to throughout the year at least. The presentations throughout 2017 (and 2018) did their job in pushing games.