The interesting question here would be if "Tell all their 3rd party publishers to stop publishing on Steam" would be legally OK or if it would result in a lawsuit against Nintendo because of e.g. anti-competitive behavior.
Retro games are in jeopardy of being lost forever because publishers/developers actively work against making it possible to preserve game history.
If that doesn't do harm to the common good, I don't know what does.
Also I doubt they would care about the legality in the slightest. It's not like they would send a press release about it.
It would be an one-time, in-person, strictly private meeting between the respective publishers and executive management. Like the one where they're shown a new console and talk about renewed licensing terms for the platform.
Nintendo is straight up evil. US courts have told them in direct language that they are wrong and emulation is fine, but they don't give a fuck and the US seems against preventing big money entities from harassing people with lawsuits or fixing the goddamn court system to not cost you a million dollars when you are unambiguously in the right.