> There are murders so why make it illegal.....
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.
Your Harry Potter case is a good illustration.
The world Harry Potter universe clearly borrows heavily on past literature - whether that be tales of goblins and wizards or tales of boarding school.
So much so ( and the fact that she is very rich ) she has been sued multiple times for plagiarism - all of them failed.
Is it really beyond the wit of man to come up with something similarly new?
In terms of fan fiction - is it ok to do one, and promote it on the internet, that has incest themes or paedophilia? These potentially reflect back on the original - and surely the creator should have control here.
There is always the option of coming to an agreement with the original author - that's how films get made - but frankly there is an almost infinite creative space - I don't see why people's lack of creativity needs to be promoted above the rights of an existing creator.`
Note in the example you quote - JK Rowling has allowed the non-commercial publishing of that and other fan fiction - just blocked independent commercial exploitation. So in the case you made the existence of copyright didn't stop Harry Potter and the Methods of Rationality.