I'll give an example to make the question more clear.
Recently a person uploaded and made public a clone of Super Mario, able to run on Commodore 64. This is clearly not just a pirate copy of the old game, but it could contain such a thing. Regardless, even if he coded and drew everything from scratch, he still violated Nintendos copyright. So say Nintendo which promptly 4 days later had enforced their DMCA takedown and the content was pulled.
Now, comments I see many places online relates to how un-nice that was of Nintendo. Typically it ends with something like "- It's not like they're losing any money over this".
Now, is this comment actually true? Let's assume that Nintendo doesn't sell this particular game any more so from actual sales, let's assume the meaning of that comment is true. They won't lose a sale to this clone because they're simply not selling the game.
I recall having read that if a copyright holder doesn't enforce his copyright claims, for things like older content in this case, they risk having problems enforcing copyright claims on new stuff.
So my question is this, is this true? Will a copyright holder get some kind of problems enforcing copyright on new, still-being-sold goods, if they're lax on protecting older goods?
In other words, if Nintendo looked the other way in this case, could they risk having problems enforcing DMCA takedowns and copyright claims on copies or clones of Super Mario Bros on Switch (as an example), a game which still sells quite nicely I suspect?