Your mistake is thinking about brands like Charles Dickens and Disney. Of course they make money, and of course they would continue producing work if copyright was only 20 years.

What you have to think about is the author who is not famous, but who makes his living off of selling books. Not the millionaires, the ones with a regular salary. They often sell old stories to short-story collections, for example, for a few thousand dollars. If this happens often enough, they can continue the life of book-writing. If not, we'll have less authors.

At the end of the day, if less money goes into writing books, we're going to lose some authors who won't be able to afford writing. Some great authors will also never be discovered, because they couldn't afford to keep writing for long enough.

It's a balance, and I'm not saying that the balance is right at the moment. But it's very disingenuous to say "Disney will still make movies", because they're not the only ones around.

Note: I talk about books, but the same is true for indie bands, indie software developers, etc.