The downsides of murder clearly outweigh the upsides. The fact that we're debating here shows that's not the case for copyright.
> ...somebody having the rights to a cartoon mouse in perpetuity isn't actually holding society back is it?
People want to adapt that mouse into their own works, as they have done with stories since stories started being told. They do so even if they know they're breaking the law. That alone should be reason to reconsider copyright.
We all stand on the shoulders of giants, and reusing facets of modern stories isn't somehow less creative than reusing old stories or reusing lived experiences. You can take the general story beats of a popular work, strip out all copyrightable elements, and end up with something wholly derivative. You can also build on a known work and end up with something that's fresh, creative, maybe something that engages readers/watchers/listeners of the base work in a way that nothing fully original could. Take Harry Potter and the Methods of Rationality; it clearly reuses another work's characters and setting, but it would be worse off without it.