Nintendo's main profit driver is software sales. Their 3rd party royalties are basically nonexistant at this point, and while Nintendo normally does make a profit on hardware sales, it's hardly as much as they make from software.
If the money they'd get from increased software sales from launching on a tremendously bigger userbase outweighs what they lose from 3rd party royalties and hardware profit, then they should go third party. Neither us nor Nintendo knows if it will though.
Nintemdo are in a peculiar position among the three platform makers because of exactly this. MS and Sony's primary profit drivers are 3rd party royalties and subscription fees, both of which are things that they need to maintain their own platform to keep. Nintendo's main profit driver is in no way tied to their hardware, and could potentially benefit a lot from a higher userbase. This is why Nintendo going third party is in no way as silly as someone make it out to be.
To end it, I'll say this: It's not about wether or not they make money at all as a platform maker, but if they can make more as a 3rd party.
*raises flame shield*
