I don't know if it's copyright issues or trademark. From my understanding (in America anyway) copyright holders are eternal and can do as they please, either sue and shut down a project, or wait and let them do whatever. Even going so far as waiting until it's hugely popular and then stealing all the assets. Generally called copyright trolling

But I think what people are talking about here is actually Trademark, rather than Copyright. My understanding is that to hold a Trademark for something, you must actively support that trademark and protect it from others. If someone uses your trademark, you're obligated to take action or lose the trademark by default. This is what King was supposedly doing to The Banner Saga and other games. Protecting a trademark (one they shouldn't have tried to trademark in the first place).

Maybe Konami doesn't have MGS trademarked for some reason, so they have no obligation to attack fans to protect the trademark