If let's say Star Wars falls out of copyright tomorrow, economically that has two effects. One, Disney loses a ton of future revenue. Two, countless Disney other people create derivatives of Star Wars, and they make money from those. Competition is increased.
So the expiration of a copyright results in a sharing of the wealth. The wealth generating potential along with the creative potential is passed along to all members of society. Our culture becomes richer and deeper. A great example of this is all the works that build on the mythos created by HP Lovecraft, one of the last great ones created before Congress started indefinitely extending copyright. Lovecraft wrote great literature and some of the authors that built on his world are fantastic as well, I'm sure they've come up with countless ideas he never considered. But as long as Congress keeps on extending copyright, nothing we create today will ever become like that.
There is of course an important question about what is fair and how long a copyright should last. Most people these days agree that it should last for at least the author's lifetime, maybe long enough to benefit their kids and grandkids as well. But the status quo is basically permanent copyright which prevents substantial creative and economic benefits to society.
Three, the derivatives are made and Disney starts marketing "Disney's Star Wars" which continue to be the high-demand (and high-value) versions. The situation is unchanged.
For example, you can currently buy The Little Mermaid in non-Disney form[1], but Disney's version is what most people want.
[1] - https://www.amazon.com/s?k=little+mermaid+Hans+Christian+And...