Going third party willingly would produce a death spiral. If Nintendo stops selling their own hardware, then they would need to significantly reduce their workforce damaging employee morale. They would be viewed as a fallen company and thus investor confidence would be lost.
There is no reason for Nintendo to go third party unless they have no other choice. Once they go third party there is no going back. In addition, every single hardware maker that stopped selling hardware were either bought or went defunct.
Nintendo does not desire to be the next Atari or Sega.
Just like Nintendo was viewed as a fallen company by investors when they announced partly going third party by entering the mobile market?