I'm really not sure it's much of a problem.
Sherlock Holmes is in the public domain, and as far as I know, it's been fine. That might be why there have been so many movies and shows based on the character recently.
Imagine it's 2024 and the original Mickey Mouse cartoon just entered the public domain. How does this impact Disney?
-Anyone could watch Steamboat Willie for free (assuming it's online somewhere). I don't see this harming Disney.
-Another big studio like Warner could make a cartoon with the original Mickey. I doubt any of the big studios are even interested in doing so, but if they do, I don't think it will affect Disney's revenue.
-Small studios and independent artists could use the original Mickey character. I think if anyone does this, it's more likely to help than hurt Disney, by boosting Mickey's profile.
-Other manufacturers could make Mickey Mouse merchandise. This is probably the biggest direct harm to Disney, but I don't think it'll make much impact on their revenue.
Some things people couldn't do:
-Use the Mickey Mouse logo. It's protected by trademark.
-Make a sequel or spin off of a modern Mickey Mouse product. Another studio couldn't just make Epic Mickey 2 since Epic Mickey (2010) is still protected.
I'm not sure Disney makes much money off of Mickey compared to other properties like Star Wars, Marvel, Frozen, etc. Mickey's popularity seems to be waning. Apart from a few video games, he hasn't been in much recently. He has a TV show that did fine but isn't particularly popular.
Okay, but what about other popular properties? The first Harry Potter book was published 24 years ago. What if it were in the public domain?
-J. K. Rowling's net worth is estimated to be over a billion USD. She'll be okay.
-People would still buy new books written by J. K. Rowling.
-Only her earliest books would be in the public domain.
In most jobs, you can't expect to work for a few years and be set for life. I'm not sure why it should be different for authors (and it usually isn't). If your book won't make enough money for you to retire after 20 years of sales, you'd either have to write another book, or get another job. That's already the case for the vast majority of authors.
The trick to dealing with a 20 year copyright term is to keep making new works. Which is exactly what copyright was supposed to encourage.