If you know that a system will get essentially all third party console games (on top of the first party titles), that is a level of assurance that's not insignificant when predicting a portion of sales for a system, if there's a prescience.
Nintendo like to change their consoles pretty drastically. Wii that missed out on most major third party releases still managed to reach 100m+. While WiiU with arguably one of the best first party lineups reached 15m.
If Nintendo release a Switch 2 that's fairly identical to Switch 1, then there's a prescience for that kind of system, library and audience. And it would be easier to predict some of Switch 2's long term success.
Right. But there are fewer of them to anticipate. And any deviation has a bigger potential impact.
And how many more people will these games bring in during a systems later years?
If you're gonna keep bringing up past systems like Wii and WiiU, it should also be mentioned that Wii's first part support dried up in it's later years, and was more focused on riding it's blue ocean strategy which also dried up quickly. WiiU on the other hand started off with barely any first party support for it's first couple years, and along with it's terrible marketing never got off the ground.
There's no mystery or confusion this time about what kind of support Switch will be getting. There's no teams making games for a seperate system now, so all their relevant titles are expected and coming to Switch. For anyone to think there'll suddenly just be a lack of first party games in the next year or two, you'd have to believe Nintendo's teams are either just working on nothing, or stockpiling titles for another system launch, both which run contrary to every piece of evidence we have.
You may think there are fewer games to anticipate, but it helps that relevant titles on Nintendo's system hold more weight over time both before and after release. Using your example of The Last of US 2, fans might get upset and need other games to help keep them busy, because since the last title released interest in that series wasn't all that high, until the next game got announced and started to get closer to release. Whereas to use something like Mario Kart for an example, there are still threads right now discussing if they should even release a sequel on Switch, because interest in MK8D is still so high.
Other easy games on Switch I could use as an example are Breath of the Wild, Mario Odyssey, Smash Bros. Ultimate, Splatoon 2, Animal Crossing and even the lazy Pokemon games. While Breath of the Wild already has a confirmed sequel on the way soon, and we know there'll be more Pokemon, none of those games necessarily need sequels to maintain interest because they keep selling. Even while notoriously maintaining their release retail price. This isn't a direct reflection on the quality of all those games, as proven by Pokemon SW/SH, but simply a reflection of the sales trends and how many people still want to buy these games years later.
On the other side, when Breath of the Wild had several delays and eventually even got changed to a cross-release on WiiU + Switch, fans were still willing to wait it out. Anticipation rather actually grew, helped by the fact it would no longer be tied to a dead system (stark contrast to the Switch). Whether Breath of the Wild 2 gets released these holidays or mid next year, it hasn't dulled anticipation for it, nor has it harmed Switch's momentum, with systems selling out as quick as they can be shipped.
Whether Mario Kart 9 gets released in the next couple of years for Switch, or held back for their next system, people will still be buying and playing Mario Kart. And if they aren't working on Mario Kart 9 right now for Switch, they're definitely working on some other relevant IP for Switch. With Nintendo only focusing on the one System Switch, and making several declarations about wanting to support the system for an extended time, there's no confusion that all their resources are being focused here, and there'll continue to be a steady stream of relevant titles in due time. Along with the several already relevant games that continue to sell and maintain high interest.