The Igros Moshe, which is the only source you cited, seems to clearly base it on hasagas gvul. In fact he writes it is midas sdom for the producer to be makpid on copying if it causes him no loss. i.e. If it is no longer being sold, it is okay to copy it.
Rav Elyashev says there is no concept of intellectual property in halacha. If I recall, he said that dina dmalchusa precludes copying. But that means that if, for example, Congress passes a law that makes a copyright expire after five years, you could halachicly copy anything over five years old. Also, most poskim say dina dmalchusa doesn’t apply to dinei maminos bein adam lchaveiro.