The founders of the US intended copyright to last for 14 years, with the ability to renew for a single extra 14-year extension. Like patents, copyright was intended to expire quickly in order to serve the good of the public. Thank Disney for fucking that up for everyone. You have refuted your own argument.
But specifically a lot of us have an issue with the fact that beginning in the 20th century, copyright durations have been increasing so quickly that it's starting to look more like copyright is becoming permanent.
The founders were well aware of the negative societal effects of monopoly. We are well aware of them and suffering under several forms of monopoly today, many of which are enforced through copyright. The founders chose to grant a LIMITED and TEMPORARY monopoly to copyright holders, not an UNLIMITED and PERMANENT one, because of this. The eminently reasonable and middle of the road position is that we should reduce the duration of copyright, restoring it to its original intent. This corrects one of the general class of pro-monopoly errors we have made which have increased wealth inequality and damaged our society.
It is truly extraordinary how much damage Disney has done (with government as a willing collaborator) against the original intent of the founders and the will of the people.
Nor encorugae assassinations by having stuff go public domain immediately upon death.
We have copyright "To promote the Progress of Science and useful Arts"
If our current implementation is not an optimal way to achieve this goal, then it needs to be re-evaluated.
FWIW, this is also why we have copyright. In fact, this is explicitly why we allow copyright in US.