"Are you going to tell them they have no right to their own work anymore 20 years after they had one of their most successful releases?"

In a word, yes. The entire purpose of copyright is to enrich the commons by granting a limited term monopoly. Without that purpose in mind, copyright loses legitimacy.

It's a government granted right, and, in the US, there is specific authorization in the constitution to make such a grant. The term could be set to any term, and there would be no underlying constitutional of inherent human rights case against such a change. Unlike actual rights, copyright is whatever governments say it is.