The problem comes in when unbreakable DRM is introduced, including DRM that depends on an external server to provide access to licensed content. If the copy protection remains uncrackable after the copyright term expires, or if the server it depends on is no longer available, it amounts to theft from the public domain, pure and simple.
Content producers should be forced to choose between legal protection for their copyrights and technical protection. They should never have been permitted to claim both.
One doesn't need DRM to shut down a server and keep the assets/ net code from seeing the light of day. Plenty of MMOs have shut down with years of people's lives down the drain. Even if you were right about how companies with DRM shouldn't obtain copyrights, I don't see how uncrackability of DRM would be a significant limiting factor when an off switch and forgotten source code is just as capable in doing the same. It wouldn't solve the fundamental issue of, to quote you, theft from the public domain.
That's a separate question (but equally worth addressing). The point here, though, is that copyright is an explicit bargain with the public domain, and DRM makes it a one-sided one.
Content producers should be forced to choose between legal protection for their copyrights and technical protection. They should never have been permitted to claim both.