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Of course someone could create a NES copier.
The mappers could be emulated with some programmable logic,
now that all the mappers are known.
There are no real technical barriers to making a NES copier,
just that nobody wants to spend the effort because it's
expensive and time-consuming to build stuff like that and
you couldn't really make any money trying to sell it, so
there's no point, unless you're just making it for yourself.