Time for the GBAtemp quick intellectual property legal educational brief as it likely applies here to this. Short version. Keep sharing the tapes, figure out how to make diffs and compile things.
There are various types of intellectual property, however the big three are
1) Patents (not really going to apply here as it is mostly for actual real world inventions, though the US and Japan do something most of the rest of the world considers unthinkable and patent software). Patents are the thing that is around 20 years, though some fields are shorter (medicine usually) and there are odd things like submarine patents if you want to go there.
is long but provides a nice overview of patents in the US which is good enough for most of the world. Same conference and author has one of
software patents as well if you do still find yourself curious.
2) Copyright (this is the work itself, this being a means of creative expression fixed in a tangible medium). Chances are pretty good nobody here today will live to see Mario 64 out of copyright.
Video on copyright lengths (nothing varies between types of content here, any variation in this sort of thing is usually part of patents. If something is within the better part of a century or more still the only reason for it to be out of copyright is either failure to register (tricky) or someone dedicated it to the public domain (maybe also CC0 which is functionally the same thing
https://creativecommons.org/share-your-work/public-domain/cc0/ ).
3) Trademarks. Names usually for goods and services you apply to an office to get registered to you for certain fields for the most part, designed so someone else can't make a product of the same name and confuse your consumers. Trademarks are the things you are usually seen to have to actively defend from widespread abuse (and given this has been plastered all over the gaming press and press at large you are unlikely to be able to claim ignorance when it actually matters).
https://www.eff.org/deeplinks/2013/...-require-companies-tirelessly-censor-internet . Note this is trademarks, copyright is different and mistaking the two is much akin to sega playstation in terms of degree of screw up. Trademarks are essentially infinite if the current owner pays a trivial sum every 20 or so years (varies by location).
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Copyright itself usually uses phrases like substantial similarity. For code there are exceptions as there are limited ways to do things, however as this was a direct decompilation of the original work it falls into a category called derived works and is nowhere near coincidence. This would probably still apply even if they stripped all the levels, sound and whatever else out of the code (something like it there, as are some graphics) and got you to feed something a copy of the ROM somewhere along the line as the code itself (in one form or another) would still exist.
The closest I have for anything like a defence is interoperability of software (one of the big things that allows emulators to exist) might well be able to apply here, however that is a defence you would likely have to make in court and I would never put any money on it being dismissed as a summary judgement here like you might for some fair use type setups.
Copyright does not have quite as strong a duty to defend, however there is something of one. Nintendo actually had a fairly notable case in it for Donkey Kong way back when (their main lawyer had the surname Kirby, it is not a coincidence).
https://law.justia.com/cases/federal/district-courts/FSupp/615/838/1515073/
That said for copyright purposes they reasonably could ignore it (though trademarks would still be in play). However they are well within their rights and what we typically see in business, especially Nintendo's business model (I assume we have all seen the "our way is the only way to play the good stuff, have some nostalgia on our terms" model over the last 20+ years), to actually go after this.
Re: it is a 20 year old game. Except they released it again on the DS and later on virtual consoles. Not that such things matter as again most here will never live to see it end for even the first Mario 64 release version, barring radical life extension or even more radical reworking of copyright laws.
Continuing with the videos thing if this sort of thing interests you I highly recommend
and the two lawyers featured in that video for general copyright assessments.
On the other hand, taking down YT videos is pretty assholistic of them, because the videos do not infringe on their rights.
Heck, DMCA gives companies too much power over what they can do with anything that has any of their IPs mentioned in it. It's basically the same story as with Ninty taking down hacking related vids, which in no way infringe on their IP in any imaginable way...or how Apple files copyright claims on Hackintosh videos (yes, sure, the people who are making the video are breaking the EULA, but that still does not infringe on any copyright).
The video thing I would probably go with trademark for. If someone sees Mario 64 PC version then they might assume it is a legit product and cause confusion in the marketplace, or even obtain said product and call their support or something. Basically the exact thing trademarks are supposed to prevent and sort of thing you see in almost every court case here that is not fair dealing related or destruction related.
If you go reading the likes of the video usage agreements from Nintendo, Microsoft and the like you will tend to find phrasing like you can't run hacked versions of the game, or sometimes even name your video as the name of the game for similar reasons.
As for hacking videos. That does get into more tricky areas as there are justifiable reasons to hack things (or indeed be free to do so), however there are provisions within the DMCA to prevent some things here.
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