I might be wrong but I believe you're overvaluing the power Nintendo really has and how gullible PC players are.Nintendo would make a killing if they start porting their old classic titles to Steam at like 15-20$ each. With Steam cards and achievements they would bring in so much additional revenue because you know Steam/PC community will eat that shit up. And they have hundreds n hundreds of games they can do that with.
But really if Nintendo ever did such a thing there is no chance they would release games on Steam. They are the type of company who will make their own storefront (as shit as it would be) and they would get away with it. They would do really well too unlike most of these other Steam competitors.
There's zero chance old games at 15$-20$ each would sell on PC. Compilations at that price absolutely, for instance a NES+SNES Mario collection or 3D Mario collection with 3 or 4 games, otherwise they'd bomb hard. The PC player base has a well recorded history of voting with their wallets and not allowing such rampant exploitation like their console counterparts, so if Nintendo tries to pull off the same type of shenanigans when releasing old games as they do with their consoles they'll crash and burn.
There's also the fact that, outside of a few gigantic names like Mario or Zelda (to some extent), there really aren't that many bankable IPs that PC players would flock to get. Outside of the 2 I mentioned I can think of Fire Emblem and Metroid and those are a huge 'maybe'. Kirbi, starfox, yoshi, smash, etc wouldn't sell on PC outside of a very small niche of players. I don't know how big the party games playerbase on PC is so Mario Party might be a 'maybe' also but even then it's banking on the mario name more than anything else.
As for they having their own storefront, I believe it can happen but I don't see it succeeding. It's been tried in the past by big publishers with much more PC-centric audiences and IPs and failed so why would it work for nintendo? Again, I don't see the PC crowd flocking towards nintendo stuff just because it's nintendo and I don't see their console centric fans switching to PC just because they can, they're loyal to their box and will remain so till the end.
It's a fun topic but completely irrelevant because it's practically impossible that nintendo releases games outside of their ecosystem, it would completely destroy their business model.