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

[Soundwave]

The reason this idea is persistent is aside from a few years with the Wii where they added casuals for a little while, they have been losing marketshare ever since the NES.

NES - 63 mill
SNES - 49 mill
N64 - 33 mill
GameCube - 22 mill
Wii U - 16-17 mill finish (projected)