Thank you all for your responses, it has been very helpful in clarifying this for me.
I don't understand the aggression from Tom Sloper, as I am clearly new and asking these questions in ernest. I am just a hobbyist who has no experience or knowledge in these things, and I have seen very successful games like "I Wanna Be The Guy" or "I Wanna Be The Boshy" using almost exclusively graphics from games on the original Nintento Entertainment System. I legitimately have no knowledge of what might be going on behind the scenes with these games legally, hence my question on this forum.
I don't read Toms response as being aggressive at all.
If anything, people are a little bit irritated by the amount of questions like these asked on this forum, given it should be obvious (or so you might think).
In itself it is a valid question, but the answer is rather obvious, and given you already seemed to know the answer, Tom did have a point with his first paragraph.
Now, you are free to break IP laws of course. That is basically what all these guys did. Did some get away with it? Yes, the larger a brand is, the more infringement cases needs to be handled. Given the size of Nintendos brand, the long time they have been around and the sheer importance of their legacy, even a company very aggressively going after infringers will have a tough time fighting the tides of infringing games.
That does not make it legal. Nintendo could at any time decide to go after all these games, and the best these guys could hope for is most probably a cease and desist order. Worst case they would loose a very expensive lawsuit.
Looking at some pics of the "I Wanna Be The Boshy" game, I'd say it is very unlikely they could win a lawsuit. They are clearly infringing. Given that is is rather unlikely they got permission by Nintendo to use their property for free (Nintendo doesn't do that), or license it (Again, nintendo doesn't do that), and it is unlikely Nintendo did settle (Nintendo doesn't do that, has not reason to do so and the infringer most probably hasn't got the needed cash), they most probably are just infringers yet to be discovered by Nintendos legal team.
Expect swift action should that happen. You can expect the game to be gone in an instant, at least from all the storefronts if it is there. Might linger on longer on sharing sites and whatever, but depending on how badly Nintendo wants to strike, they might go after everyone that hosts it and punish them hard.
You need to understand that many on these forums are Indie devs, or working in the Games Industry, others are serious hobbyist. Neither of them is particularly fond of people "stealing" other peoples IP, because they don't want it to happen to them (when they most probably CANNOT afford to go after infringers the same way the big studios can), and because it is just that... stealing some elses (intellectual) property.
And then there is a TON of assets and code available online that others have made free to use and shared it with the broader community. First see if you cannot find LEGAL art before you complain about not being able to proceed with your project because of missing art.
TL;DR
1. It doesn't matter how old something is. As long as it is still protected, you are infringing.
2. It doesn't matter if you make your copycat free. You are still infringing.
3. It doesn't matter if your games are successfull or not. You might get discovered and sued earlier in the former case, you are still infringing in the latter.
If you are looking for good advice on these forums, these are about all you will get:
1. Don't infringe someone elses IP, come up with your original ideas, names and graphics
2. If you are no artist, either
a) learn to live with your limitations and embrace the programmer art... its the new chic in some Indie genres
b) team up with an artist... hard to do without expierience or money to pay them, but expierience and money can be built up over time
c) become more competent at art yourself... yes, even someone with no artistic skill can do that. Might not be the next picasso, but that is hardly the target.
3. Enjoy the uniqueness your own ideas will bring to your game... I am sure your players will like that way more than the next lame Mario ripoff with original graphics and whatnot.
(4. failing all of that, search the net for free art, or buy cheap stock art. If you are searching enough, you will find practically everything you need.)