I agree with you OP but i would say :
-More mature consumers are the target compare to the Wii/DS/Wii U/3DS era (which doesn't mean kids and casual adult players won't jump in, kids like to feel mature anyway, look at call of duty lmao)
-Nintendo fans have only 1 system to buy. Might seem stupid, but once you don't have to buy 2 times a mario kart game for 2 different consoles, you have time and money for something else. I have so much more games on Switch than on my previous Nintendo consoles for that reason. I like having everything on the same device and makes me feel like buying games on it is worth it.
-I would also add the love for the system in general. People love the Switch a lot, it doesn't only sell well but people still like it a lot, which yes it's a big deal. You see so much more positive feedbacks online and it makes you want to spend money on it, from what i feel and i've read a lot on Twitter.
It's not only third party that sells more but also Nintendo games. Almost every entry on switch has been the best selling of it's franchise or will be in the future. Like even kirby that got rolled on by the press is passed like 2 million shipped. Games are just selling incredibly well on it that's all, but it's nice to see it not being limited to first party games tho.