> the underlying principle behind the GPL could not be achieved without copyright. The GPL doesn't merely require you to let other people reverse engineer your binaries, it contemplates that the distributed source code will be in the "preferred form" for working on it. There is no way to enforce that provision without copyright.

Sure there is. The GPL just used copyright as a hack to enforce on a limited basis what the creators explicitly think ought to be a universal obligation not dependent on the use of some upstream content that imposes the rule, for which the natural enforcement mechanism would be for the rules to be in law directly instead of copyright. “Copyleft” essentially implies that—its twisting what proponents see as a bad law into a tool for approximating the nearly diameteically-opposed law they wish existed in its place.