Why do people think that Nintendo could go or should go third-party?

[Soundwave]

Nintendo going third party would suck. Games like Xenoblade, Pilotwings, Punch-Out!, and probably even things like Metroid would go away.

The reason these games are made now is that Nintendo has a whole platform to support so they are forced to make a variety of games.

Make them a third party developer and they'd make nothing but Mario Kart, Mario, Zelda, and a few other IP over and over and over again because that would make business sense.

Sega had to make IP like Daytona, Space Channel 5, Shinobi, Skies of Arcadia, Streets Of Rage, etc. back in the day because they needed to support a platform ... nowadays? They don't make any of these games. It's all Sonic + a few other IP and nothing else.