Somehow I’m not that concerned. I haven’t heard of any company but Nintendo trying to assert rights to 1980s game IP, and that’s because they’re still selling those literal ROMs as part of a subscription service.
Would I port such a game and then sell it? No, because that kind of puts a target on your back. Keeping it open source and also noncommercial, I don’t think it’s ever gonna matter.
(Obligatory I’m not a lawyer disclaimer - this is a vibes based comment not legal advice. Obviously copyright is nearly infinite, in theory)
It's not because they're selling ROMs, that only accounts for a small fraction of their back library.
It's because they're assholes.
Many of their franchises date back to that era and are incredibly valuable. It's like Disney caring about Steamboat Willey all those years.
No argument on the latter, but I feel like they also have a 'practical' concern that if they very openly ignored 8-bit and 16-bit 'piracy,' it's conceivable that devices like the Anbernic and Powkiddy ones (that already exist in our timeline) would get sold more in the open and be even easier to get started with. Maybe some parents (say, millennials who want our kids to learn the kind of gaming we grew up on) would rather buy those than a $349 Switch with the $10/month subscription they sell to the selection of 80s and 90s games in Switch Online.
In our timeline of course, those are already easy to get, but those companies cater to the kinds of nerds who flash a new OS to SD cards and download collections of ROMs.
Now, do I think Nintendo actually makes that much money on Switch Online from people who (A) are only there for the retro games and (B) would jump on some "more mainstream" and easier version of retro piracy handheld? Mehhhhhh not really. But someone probably espouses that viewpoint at Nintendo HQ.