Good luck with that. Switch is on track to be the best selling systems of all time, there's little reason for them to even consider going third party any time soon.
Though Nintendo have the least reason of the big 3 to remain as an exclusive developer. They don't make a ton of money from third parties, they aren't making a ton of money from services. The vast majority of their profits come from the sale of their games unlike Sony/MS.
Out of the 246m software sold on the Switch over 130m of that is published by Nintendo. The point of having your own platform is Third Party revenue and Service revenue which Nintendo gets the least of the big three by an absolute landslide.
Nintendo's interest should lie in making as much money from their games as they can, and releasing to a wider audience may make them even more successful.
But yes Nintendo isn't going to release on other systems anytime soon, even though it makes more sense for them than it does for PlayStation or Xbox.
That 246 million figure does not include digital only titles, which make up the majority of 3rd party titles on Switch.
But even if 3rd party and service revenue/profit is lower than the others, that is still money they would be throwing away by going 3rd party, same with hardware revenue/profit and on top of that they would start giving up 30% of their software revenue to the other platform holders.
Then there is the fact that a large amount of the Nintendo userbase prefers portable gaming which is not offered by Sony/Microsoft/Steam.
One more thing is that there isnt an existing audience on PS/XB/PC for many of the genres that Nintendo games make up. How many successful $60 platformers are there on those platforms? How many successful $60 social simulators are on those platforms? How many successful fitness, rhythm games & minigame compilations are on those platforms? Will games like Kirby, Yoshi & Donkey Kong or Animal Crossing, Tomodachi & Style Savvy or Ring Fit, Rhythm Heaven & WarioWare see notable growth on these platforms?
I'm sure the mainstream franchises like 2D/3D Mario, Mario Kart, Smash Bros, Zelda & Pokemon could benefit but I'm not so sure about the dozens of other franchises.