This could be the case if we would talk about one game. But the whole Mario, Zelda, Pokemon, Kirby, Metroid and etc. Franchises are exclusive on Nintendo and they are objektiv good games (maybe pokemon has fallen of but it's still a money grab) you don't have that many exclusives on PS4 and 5 and Xbox one and X/S together. And it shows. The Switch and Switch light still outsells Sony and Microsoft consoles.

My friend group got switches mainly because of pokemon. But I dropped it for other games because Gamefreak underdeliverd IMHO.

Also we know that Nintendo is overprotective about their IPs. I guess they think that if they solely sell their games on their closed system they can decrease the chance of pirating. And it's not far fetched because you need a decent PC to emulate and play switch games. Also know how on how to set it up. (which why I belive, that the onemillion is just a fantasy number from Nintendo in this case)

Of course they would make a fortune selling their old games on pc. But it's Nintendo we talking about, they are super conservative about things like that.