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.
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 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 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.