I think you are giving Nintendo's Switch success too much credit.
And that is why i said that Switch might suffer down the road if it can't broaden it's offer with 3rd party titles,
Yes, it is true that Switch had the best lineup ever for a console in year 1. Is that even in doubt?
What history shows us is that it's difficult to support two consoles at the same time without having any glaring gaps of multimillion-selling software. But now Nintendo doesn't have to go back and forth between two consoles anymore. There is only Switch, so that makes it a lot more feasible that Nintendo alone can sustain momentum.
I concede that having third party support is beneficial, but here it is important to focus on third party games that actually matter. You make the big mistake to assume that games that are big on PS and Xbox will be similarly important for Nintendo, but that's simply not the case based on historical sales data and the already available sales data for Switch. The reason why there is such a gap in importance is that third parties specifically tailor their software to gamers who avoid Nintendo, so on the flipside you get the result that people who buy Nintendo consoles aren't showing much interest in those third party games because they weren't made for them. An obvious example of this is that an M-rating is perceived as something valuable by PS and Xbox gamers while Nintendo gamers shrug their shoulders and tell you that a game is going to be good or bad regardless of its rating; in other words, glorification of violence is little to non-existent in the minds of Nintendo gamers. Most of the M-rated games don't offer much beyond violence, that's why they fail to gain traction on Nintendo consoles. The Resident Evil IP is one of the few M-rated series that manages to provide actual substance in its gameplay, that's why it is doing notably better than other M-rated games.
Additionally, video game history shows us that during the times Nintendo pursued the third party support you ask for, Nintendo's sales levels were at their worst. Both the GameCube and Wii U flopped hard and that's no coincidence when you take the above paragraph into account. Nintendo put more emphasis on games that their customers don't care much for, so customers turned their backs on Nintendo. Should Nintendo pursue that path again, they'll inevitably rush out a Switch successor that will predictably fail, because the vast majority of Switch owners will not be interested in moving on.